Add serde_utils module with quoted u64 support (#1588)

## Proposed Changes

This is an extraction of the quoted int code from #1569, that I've come to rely on for #1544.

It allows us to parse integers from serde strings in YAML, JSON, etc. The main differences from the code in Paul's original PR are:

* Added a submodule that makes quoting mandatory (`require_quotes`).
* Decoding is generic over the type `T` being decoded. You can use `#[serde(with = "serde_utils::quoted_u64::require_quotes")]` on `Epoch` and `Slot` fields (this is what I do in my slashing protection PR).

I've turned on quoting for `Epoch` and `Slot` in this PR, but will leave the other `types` changes to you Paul.

I opted to put everything in the `conseus/serde_utils` module so that BLS can use it without a circular dependency. In future when we want to publish `types` I think we could publish `serde_utils` as `lighthouse_serde_utils` or something. Open to other ideas on this front too.

consensus/serde_utils/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "serde_utils"
+version = "0.1.0"
+authors = ["Paul Hauner <paul@paulhauner.com", "Michael Sproul <michael@sigmaprime.io>"]
+edition = "2018"
+
+[dependencies]
+serde = { version = "1.0.110", features = ["derive"] }
+serde_derive = "1.0.110"
+
+[dev-dependencies]
+serde_json = "1.0.52"