Add Gloas state builders Beacon API endpoint

[rolfyone]

PR Description

Summary
Implements ethereum/beacon-APIs#614 for Teku by adding the Gloas-only POST /eth/v1/beacon/states/{state_id}/builders Beacon API endpoint with JSON and SSZ response support. The endpoint accepts optional builder ids, filters by builder index or pubkey, and returns each builder with its derived pending, active, or exited status.

Testing

./gradlew :data:provider:spotlessCheck :data:beaconrestapi:spotlessCheck :data:serializer:spotlessCheck :ethereum:json-types:spotlessCheck :data:provider:test --tests tech.pegasys.teku.api.ChainDataProviderTest :data:beaconrestapi:test --tests tech.pegasys.teku.beaconrestapi.handlers.v1.beacon.PostStateBuildersTest
./gradlew :data:beaconrestapi:integrationTest --tests tech.pegasys.teku.beaconrestapi.beacon.OpenApiIntegrationTest
git diff --check

Fixed Issue(s)

Documentation

• I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.

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Note

Low Risk
Read-only state query API behind Gloas milestone gating, mirroring existing validator filter endpoints with no consensus or signing changes.

Overview
Adds the Gloas-only POST /eth/v1/beacon/states/{state_id}/builders Beacon API (ethereum/beacon-APIs#614), registered via GloasRestApiBuilderAddon when Gloas is enabled.

Clients may send an optional ids body to filter builders by numeric index or 0x-prefixed pubkey; omitting ids returns every builder in state. Each entry includes index, pending/active/exited status (derived from withdrawable epoch and isActiveBuilder), and the builder SSZ object. Responses support JSON and SSZ (application/octet-stream), with the usual execution_optimistic / finalized metadata.

ChainDataProvider loads the requested beacon state and maps Gloas builders; pre-Gloas states are rejected. New StateBuilderData SSZ types back the API payload. Changelog and OpenAPI fixture schemas are updated; unit tests cover the handler and provider logic.

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[gfukushima]

lgtm but we should probably wait until the api change is merged to merge this