Replace types from go-boost-utils with types from Attestant

[avalonche]

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

Switching to the attestandtio types and deprecate the go-boost-utils types would reduce maintenance for future spec upgrades (e.g. upcoming Capella fork).

[metachris]

See also:

• https://github.com/flashbots/mev-boost/pull/411
• https://github.com/flashbots/mev-boost/issues/404

[metachris]

in progress, closing this issue