Gloas: Add endpoint for builder_pending_withdrawals, builder_pending_payments

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](https://github.com/ethereum/beacon-APIs/blob/master/types/gloas/builder.yaml) covered by [/api/v1/beacon/states/{}](https://github.com/ethereum/beacon-APIs/blob/master/apis/beacon/states/builders.yaml) 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

Start by comparing types/gloas/builder.yaml with apis/beacon/states/builders.yaml and the existing /api/v1/beacon/states/{} response. Determine the endpoint paths and response fields needed for builder_pending_withdrawals and builder_pending_payments, then document them in the appropriate API and type specifications; done means the proposed endpoints and schemas are clear and consistent with the existing builder API.

Written by the indexing model from the issue text.