Submit transactions through proxy

[Radiokot]

Purpose

Fix inability to submit SDK transactions via the public GRPC node when it is being spammed.

Changes

• Add proxy endpoint to submit serialized transactions
• Submit baker/delegation transaction via the proxy
• Clean up submission status proxy endpoints
• Add deprecation warning (for visibility) to the GRPC client getter

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.