Implement SSZ union type (#2579)

## Issue Addressed

NA

## Proposed Changes

Implements the "union" type from the SSZ spec for `ssz`, `ssz_derive`, `tree_hash` and `tree_hash_derive` so it may be derived for `enums`:

https://github.com/ethereum/consensus-specs/blob/v1.1.0-beta.3/ssz/simple-serialize.md#union

The union type is required for the merge, since the `Transaction` type is defined as a single-variant union `Union[OpaqueTransaction]`.

### Crate Updates

This PR will (hopefully) cause CI to publish new versions for the following crates:

- `eth2_ssz_derive`: `0.2.1` -> `0.3.0`
- `eth2_ssz`: `0.3.0` -> `0.4.0`
- `eth2_ssz_types`: `0.2.0` -> `0.2.1`
- `tree_hash`: `0.3.0` -> `0.4.0`
- `tree_hash_derive`: `0.3.0` -> `0.4.0`

These these crates depend on each other, I've had to add a workspace-level `[patch]` for these crates. A follow-up PR will need to remove this patch, ones the new versions are published.

### Union Behaviors

We already had SSZ `Encode` and `TreeHash` derive for enums, however it just did a "transparent" pass-through of the inner value. Since the "union" decoding from the spec is in conflict with the transparent method, I've required that all `enum` have exactly one of the following enum-level attributes:

#### SSZ

-  `#[ssz(enum_behaviour = "union")]`
    - matches the spec used for the merge
-  `#[ssz(enum_behaviour = "transparent")]`
    - maintains existing functionality
    - not supported for `Decode` (never was)
    
#### TreeHash

-  `#[tree_hash(enum_behaviour = "union")]`
    - matches the spec used for the merge
-  `#[tree_hash(enum_behaviour = "transparent")]`
    - maintains existing functionality

This means that we can maintain the existing transparent behaviour, but all existing users will get a compile-time error until they explicitly opt-in to being transparent.

### Legacy Option Encoding

Before this PR, we already had a union-esque encoding for `Option<T>`. However, this was with the *old* SSZ spec where the union selector was 4 bytes. During merge specification, the spec was changed to use 1 byte for the selector.

Whilst the 4-byte `Option` encoding was never used in the spec, we used it in our database. Writing a migrate script for all occurrences of `Option` in the database would be painful, especially since it's used in the `CommitteeCache`. To avoid the migrate script, I added a serde-esque `#[ssz(with = "module")]` field-level attribute to `ssz_derive` so that we can opt into the 4-byte encoding on a field-by-field basis.

The `ssz::legacy::four_byte_impl!` macro allows a one-liner to define the module required for the `#[ssz(with = "module")]` for some `Option<T> where T: Encode + Decode`.

Notably, **I have removed `Encode` and `Decode` impls for `Option`**. I've done this to force a break on downstream users. Like I mentioned, `Option` isn't used in the spec so I don't think it'll be *that* annoying. I think it's nicer than quietly having two different union implementations or quietly breaking the existing `Option` impl.

### Crate Publish Ordering

I've modified the order in which CI publishes crates to ensure that we don't publish a crate without ensuring we already published a crate that it depends upon.

## TODO

- [ ] Queue a follow-up `[patch]`-removing PR.

consensus/tree_hash/src/lib.rs

@@ -12,6 +12,7 @@ use eth2_hashing::{hash_fixed, ZERO_HASHES, ZERO_HASHES_MAX_INDEX};
 pub const BYTES_PER_CHUNK: usize = 32;
 pub const HASHSIZE: usize = 32;
 pub const MERKLE_HASH_CHUNK: usize = 2 * BYTES_PER_CHUNK;
+pub const MAX_UNION_SELECTOR: u8 = 127;
 
 pub type Hash256 = ethereum_types::H256;
 
@@ -63,6 +64,31 @@ pub fn mix_in_length(root: &Hash256, length: usize) -> Hash256 {
     Hash256::from_slice(&eth2_hashing::hash32_concat(root.as_bytes(), &length_bytes)[..])
 }
 
+/// Returns `Some(root)` created by hashing `root` and `selector`, if `selector <=
+/// MAX_UNION_SELECTOR`. Otherwise, returns `None`.
+///
+/// Used in `TreeHash` for the "union" type.
+///
+/// ## Specification
+///
+/// ```ignore,text
+/// mix_in_selector: Given a Merkle root root and a type selector selector ("uint256" little-endian
+/// serialization) return hash(root + selector).
+/// ```
+///
+/// https://github.com/ethereum/consensus-specs/blob/v1.1.0-beta.3/ssz/simple-serialize.md#union
+pub fn mix_in_selector(root: &Hash256, selector: u8) -> Option<Hash256> {
+    if selector > MAX_UNION_SELECTOR {
+        return None;
+    }
+
+    let mut chunk = [0; BYTES_PER_CHUNK];
+    chunk[0] = selector;
+
+    let root = eth2_hashing::hash32_concat(root.as_bytes(), &chunk);
+    Some(Hash256::from_slice(&root))
+}
+
 /// Returns a cached padding node for a given height.
 fn get_zero_hash(height: usize) -> &'static [u8] {
     if height <= ZERO_HASHES_MAX_INDEX {