Proposal: replace types from go-boost-utils with types from Attestant

[metachris]

Attestant / @mcdee has been working on up-to-date types with a lot of tests: https://github.com/attestantio/go-eth2-client/tree/master/spec

I'm proposing to switch to the attestandtio types and deprecate the go-boost-utils types.

This would be one less thing to worry about, and one less repository to maintain.

[ralexstokes]

this works for me, the key things to consider are:

1. ssz library used
2. maintainability, reliability of implementation

and it looks like both sources use the same ssz library so (1) is no change and I have no concerns personally about (2)

[prestonvanloon]

We mentioned this in the Prysm release today. More context can be found here: prysmaticlabs/prysm#11717

[metachris]

work is underway, closing this for now.