Add check that block is known in on_payload_attestation_message (#5022)

Currently,
[on_payload_attestation_message](https://github.com/ethereum/consensus-specs/blob/master/specs/gloas/fork-choice.md#new-on_payload_attestation_message)
raises `KeyError` if `data.beacon_block_root` refers to unknown block.

One consequence is that
[expect_assertion_error](https://github.com/ethereum/consensus-specs/blob/1baa05e71148b0975e28918ac6022d2256b56f4a/tests/core/pyspec/eth_consensus_specs/test/context.py#L385)
won't catch the error.

Historically, block roots are validated with an explicit `assert`. The
PR adds such a check.

Author: ericsson49

specs/gloas/fork-choice.md

@@ -853,6 +853,7 @@ def on_payload_attestation_message(
     # The beacon block root must be known
     data = ptc_message.data
     # PTC attestation must be for a known block. If block is unknown, delay consideration until the block is found
+    assert data.beacon_block_root in store.block_states
     state = store.block_states[data.beacon_block_root]
     ptc = get_ptc(state, data.slot)
     # PTC votes can only change the vote for their assigned beacon block, return early otherwise