Gloas: Add endpoint for builder_pending_withdrawals, builder_pending_payments

[jshufro]

Builders are expected to have a small total size, so querying a state object with ~1MM validators is very inefficient.

We already have types/gloas/builder.yaml covered by /api/v1/beacon/states/{} which has a response containing the fields of a builder (pubkey, version, execution_address, balance, deposit_epoch, withdrawable_epoch), but not details on external state related details such as pending withdrawals and payments.

Apologies if there's a reasonable alternative to querying debug/state, and I have missed it

[nflaig]

I was planning to add these, was hoping someone asks explicitly, that makes it quite easy to justify adding them to the spec, another candidate is the ptc_window or getting the ptc for a specific slot. I don't think there is a reasonable alternative, other then querying the whole state, and historically having /eth/v1/beacon/states/{state_id}/pending_deposits and other similar endpoints has been useful and is quite easy to add for clients

[jshufro]

I have downstream applications that will benefit from the builder related ones and would definitely appreciate their addition- basically any launchpad-style tooling that can be used to manage the lifecycle of a builder (vis a vis a validator) probably wants to be able to query them directly.