Implement the shipper's gRPC API, which is currently a skeleton defined in https://github.com/elastic/elastic-agent-shipper/blob/main/server/server.go

The API specification is currently defined in https://github.com/elastic/elastic-agent-shipper/blob/main/api/shipper.proto

Acceptance Criteria:

• The PublishEvents RPC should block when the queue is full #84
• The API specification is updated with any changes or improvements that were necessary.
  • Allow google.protobuf.Timestamp in shipper Event fields and metadata #62
• A reference API client implementation exists and integrates with the server side implementation.
  • Allow the Beats' shipper output to depend on the shipper client directly. #64
  • Benchmark the shipper output's conversion from publisher.Event beats#32081
  • Implement validation for incoming events #79
• An integration test suite exists to verify that the client and server implementations interact correctly, with emphasis on error conditions: partially accepted batches, full queues, server and client side restarts, etc.
  • Make the shipper output to acknowledge batches based on PersistedIndex not AcceptedCount beats#32329
  • Beats shipper output should not wait for full acknowledgment before sending more batches #98