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/ssz/src/encode.rs

@@ -104,13 +104,21 @@ impl<'a> SszEncoder<'a> {
 
     /// Append some `item` to the SSZ bytes.
     pub fn append<T: Encode>(&mut self, item: &T) {
-        if T::is_ssz_fixed_len() {
-            item.ssz_append(&mut self.buf);
+        self.append_parameterized(T::is_ssz_fixed_len(), |buf| item.ssz_append(buf))
+    }
+
+    /// Uses `ssz_append` to append the encoding of some item to the SSZ bytes.
+    pub fn append_parameterized<F>(&mut self, is_ssz_fixed_len: bool, ssz_append: F)
+    where
+        F: Fn(&mut Vec<u8>),
+    {
+        if is_ssz_fixed_len {
+            ssz_append(&mut self.buf);
         } else {
             self.buf
                 .extend_from_slice(&encode_length(self.offset + self.variable_bytes.len()));
 
-            item.ssz_append(&mut self.variable_bytes);
+            ssz_append(&mut self.variable_bytes);
         }
     }
 
@@ -125,13 +133,6 @@ impl<'a> SszEncoder<'a> {
     }
 }
 
-/// Encode `index` as a little-endian byte array of `BYTES_PER_LENGTH_OFFSET` length.
-///
-/// If `len` is larger than `2 ^ BYTES_PER_LENGTH_OFFSET`, a `debug_assert` is raised.
-pub fn encode_union_index(index: usize) -> [u8; BYTES_PER_LENGTH_OFFSET] {
-    encode_length(index)
-}
-
 /// Encode `len` as a little-endian byte array of `BYTES_PER_LENGTH_OFFSET` length.
 ///
 /// If `len` is larger than `2 ^ BYTES_PER_LENGTH_OFFSET`, a `debug_assert` is raised.