Remove ethereum_ssz dependency

[eserilev]

Motivation

Lighthouse is currently migrating our code base and relevant dependencies to alloy_primtives. During the migration we've come across some circular dependency issues between the ethereum_ssz and alloy_primitives crates.

Solution

This PR aims to remove the optional ethereum_ssz dependency as well as the ssz feature flag.

I have an open PR here that implements ssz Decode and Encode for the alloy generic types Bytes and FixedBytes<N> in ethereum_ssz

PR Checklist

• Added Tests
• Added Documentation
• Breaking changes

[mattsse]

nice,

in order to transition we need to coordinate releases,
we first need to merge this pr, then do a new alloy-core release, followed by an ssz release so we can also update all dependent crates.

so we can merge this once ethereum_ssz is also ready to publish

[michaelsproul]

How about this for a release strategy?

1. Release ethereum_ssz v0.6 which depends on alloy-primitives 0.7.7
2. Release alloy-primitives v0.8 which does not depend on ethereum_ssz at all
3. Release ethereum_ssz v0.7 which depends on alloy-primitives v0.8

We can try to make the timing quite tight between (2) and (3). I will try to get (1) out ASAP.

[DaniPopes]

Sounds good, although I'm not sure if you need (1), but up to you