op-batcher: use one BatchAuthenticator reader, bound and code-checked once at startup

[shenkeyao]

Closes #503.

This PR:

• Follows up on Espresso 3b: TEE batcher (re-hosted) #459 (comment) to add batchAuthenticatorReader and tests.

Key places to review:

• op-batcher/batcher/batch_authenticator.go
• op-batcher/batcher/batch_authenticator_test.go

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