Add SSZ list types for gloas API request bodies

[KaloyanTanev]

Adds generated SSZ list types for the two gloas API request bodies that are bare lists on the wire, mirroring api/v1.Blobs:

• apiv1gloas.PayloadAttestationMessages — List[PayloadAttestationMessage, PTC_SIZE] (POST /eth/v1/beacon/pool/payload_attestations)
• apiv1gloas.SignedProposerPreferencesList — List[SignedProposerPreferences, (MIN_SEED_LOOKAHEAD + 1) * SLOTS_PER_EPOCH] (POST /eth/v1/validator/proposer_preferences)

Both are registered in api/v1/gloas/generate.yaml, so dynssz-gen emits the MarshalSSZ/UnmarshalSSZ implementations (alignment and list-limit checks included). The http client's private proposerPreferencesList and its hand-written marshal loop are replaced by the exported type, so a beacon node built on this library (charon) decodes these bodies with the same generated code the client uses to encode them.

Note: the generated list types have pointer receivers, so SubmitProposerPreferences passes the list by pointer to marshalRequestBody. This is only exercised by the http tests in live mode (HTTP_ADDRESS set), which the fork's CI currently does not run; verified locally against the Platåberget beacon node.