Added test cases demonstrating external-signer voluntary exit

[rolfyone]

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

New test cases have been added to VoluntaryExitCommandTest.java to demonstrate the functionality of external signers for voluntary exits.

• Updated teku/src/integration-test/java/tech/pegasys/teku/cli/subcommand/VoluntaryExitCommandTest.java to include new test methods for external signers.
• Added mock server configurations to simulate external signer responses.
• Enhanced existing methods to remove redundant parameters and handle edge cases like signer unavailability and signature validation.
• Ensured tests cover scenarios such as successful exits, rejected exits, and future epoch handling.

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