Add support for phase0 and electra in PostAttesterSlashingV2

[gfukushima]

PR Description

This PR fixes the PostAttesterSlashingV2 API to present a spec complaint with the oficial API spec supporting oneOf AttesterSlashingPhase0 or AttesterSlashingElectra

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.

[greptile-apps]

PR Summary

This PR updates the PostAttesterSlashingV2 API to support both Phase0 and Electra milestones, ensuring compliance with the official API specifications.

• data/beaconrestapi/src/integration-test/java/tech/pegasys/teku/beaconrestapi/v2/beacon/PostAttesterSlashingV2IntegrationTest.java: Updated to support Phase0 and Electra, using TestTemplate for parameterized tests and improved error handling.
• data/beaconrestapi/src/integration-test/resources/tech/pegasys/teku/beaconrestapi/beacon/paths/_eth_v2_beacon_pool_attester_slashings.json: Modified JSON schema to include a 'oneOf' clause for AttesterSlashingPhase0 or AttesterSlashingElectra.
• data/beaconrestapi/src/main/java/tech/pegasys/teku/beaconrestapi/handlers/v1/beacon/MilestoneDependentTypesUtil.java: Added headerBasedSelector method for schema selection based on HEADER_CONSENSUS_VERSION.
• data/beaconrestapi/src/main/java/tech/pegasys/teku/beaconrestapi/handlers/v2/beacon/PostAttesterSlashingV2.java: Refactored to handle both Phase0 and Electra versions using SerializableOneOfTypeDefinition and headerBasedSelector.
• data/beaconrestapi/src/test/java/tech/pegasys/teku/beaconrestapi/handlers/v2/beacon/PostAttesterSlashingV2Test.java: Updated test cases to support new attester slashing types and consensus version headers.

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[rolfyone]

LGTM