Add User Agent to Builder's headers

[zilm13]

PR Description

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

Adds optional User-Agent and standard Accept/consensus-version headers to builder REST calls and updates RestClient.getAsync to accept headers.

• Builder client (RestBuilderClient):
  • Introduces buildHeadersWith(...) to construct headers (optional User-Agent, Accept, and Eth-Consensus-Version).
  • Applies headers to status, registerValidators (SSZ/JSON), getHeader, getPayload, and getPayloadV2 requests.
• REST client API:
  • Changes RestClient.getAsync signature to require headers and updates OkHttpRestClient implementation accordingly.
• Tests:
  • Update OkHttpRestClientTest to pass headers for GET calls.
  • Update RestBuilderClientTest to reflect new postAsync signature and verify presence/absence of User-Agent and other headers.

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