Add builder_pending_withdrawals and builder_pending_payments endpoints

[markolazic01]

Motivation

Make a staked builder's unsettled payments accessible to the builder and external viewers.

Description

Closes #636

Adds two endpoints to read the Gloas builder payment fields from a given state:

• GET /eth/v1/beacon/states/{state_id}/builder_pending_withdrawals
• GET /eth/v1/beacon/states/{state_id}/builder_pending_payments

Both return the state field as is, following the pending_* and proposer_lookahead endpoints, with JSON and SSZ responses and a 400 for pre-Gloas states. The item types BuilderPendingWithdrawal and BuilderPendingPayment already exist in types/gloas/builder.yaml and remain unchanged.

[nflaig]

I am gonna sort the changelog before the release but we should add entries to the bottom, sorted by pr number but it's always a bit annoying right now with merge conflicts, so keeping it here now it fine

[markolazic01]

got it, I was not sure if I should put them to the top or the bottom

[nflaig]

after the release can explore #579 or other solutions, ideally we want a solution that is easy to understand for someone that puts up a pr and also doesn't cause merge conflicts between prs

[markolazic01]

I saw that one, I can spend some time on that if you'd like.

[nflaig]

rather low prio right now, we can resolve this after the release

[chong-he]

Do we want to add Gloas.BuilderPendingPayment and Gloas.BuilderPendingWithdrawal under components: schema:?
(some lines below in this same file)

I see that previous PR added the type in the components and schema section, for example: https://github.com/ethereum/beacon-APIs/pull/512/files#diff-962ea34dc6ec0e65d4e1d6c8682e97c547e6e0d2f1168925210ff7ee47ef6ce0R437-R438