New: Experimental QCS Client

[kalzoo]

Description

This PR includes changes needed to incorporate an experimental, gRPC-based client in place of the current rpcq-based client. These changes would involve breaking changes to the existing QuantumComputer and other interfaces, and due to expected instability, are corralled under the experimental subpackage for flexibility while still making them available to customers.

Some features of both gRPC and the new API make the new experimental QuantumComputer and related classes much simpler. Notably, they are also built with asyncio from the start.

Checklist

• The PR targets the rc branch (not master).
• Commit messages are prefixed with one of the prefixes outlined in the commit syntax checker (see pattern field).
• The above description motivates these changes.
• There is a unit test that covers these changes.
• All new and existing tests pass locally and on the PR's checks.
• Parameters and return values have type hints with PEP 484 syntax.
• Functions and classes have useful Sphinx-style docstrings.
• All code follows Black style and obeys flake8 conventions.
• (New Feature) The docs have been updated accordingly.
• (Bugfix) The associated issue is referenced above using auto-close keywords.
• The changelog is updated, including author and PR number (@username, Test ignore #1234).

[ameyer-rigetti]

Could you not update Program itself? Going from non-optional to optional should be backwards compatible

[ameyer-rigetti]

Though I guess could result in a lot of refactoring to satisfy mypy and other use cases

[kalzoo]

Part of my intent in this PR is to not have to change anything in the main library for simplicity, with this being almost a "draft" for the next major version (after a long period of iteration & feedback)

[ameyer-rigetti]

That makes good sense 👍

[kalzoo]

This work is being superseded by v4