Re-enable serialization of CircuitOperations

[95-martin-orion]

This is a combined roll-forward of #4336 and #4344. Part of #3634.

Additional changes in this PR that were not in the originals:

• Test for backwards-compatibility of processor.get_device() (verified to fail with only Allow CircuitOperations in cirq.optimized_for_sycamore #4336 and Enable CircuitOp serialization. #4344).
• Add *_WITH_CIRCUITOP gatesets to each processor proto. The existing gatesets are unmodified, so backwards-compatibility is maintained.

Next steps:

• Push changes to servers. This remains compatible: checks in get_device only care that the local gatesets can be found in the processor proto, and the new proto is a superset of the previous proto.
• Clean up non-circuitop gatesets and re-push. Removing the old gatesets is a local change, only affecting the internal naming of gatesets (e.g. XMON vs XMON_WITH_CIRCUITOP)

Known weirdness:

• Serialized gate_set_names will permanently have a trailing _co after this change (e.g. 'xmon_co' instead of 'xmon') The only way around this is to double the number of steps, at which point a temporary breakage seems preferable.

[95-martin-orion]

Discussed offline with @maffoo - there is a preferred process for handling backwards-compatible changes to gatesets which we will implement instead of this change. Closing in favor of that method.