Add tags to cirq.FrozenCircuit

[tanujkhattar]

Fixes #6265

Things left to do as a follow up

• Update circuit diagrams for cirq.FrozenCircuit to print the tags if they exist
• Tags on FrozenCircuits are not automatically "imported" to the operation level when a frozen circuit get's wrapped in a circuit operation. I think we should leave this as it is for now and, if a need arises later, we can update the interop methods to propagate the tags.

[pavoljuhas]

Looks good modulo few nits.

[codecov]

Codecov Report

Patch coverage: 95.13% and no project coverage change.

Comparison is base (ed26d2f) 97.60% compared to head (7d8607c) 97.61%.
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #6266    +/-   ##
========================================
  Coverage   97.60%   97.61%            
========================================
  Files        1116     1116            
  Lines       95848    95973   +125     
========================================
+ Hits        93556    93680   +124     
- Misses       2292     2293     +1     

Files Changed	Coverage Δ
cirq-google/cirq_google/api/v2/sweeps.py	100.00% <ø> (ø)
...rq_google/cloud/quantum_v1alpha1/types/__init__.py	100.00% <ø> (ø)
cirq-google/cirq_google/api/v2/run_context_pb2.py	35.48% <12.50%> (ø)
cirq-core/cirq/circuits/circuit.py	98.43% <100.00%> (+<0.01%)	⬆️
cirq-core/cirq/circuits/frozen_circuit.py	99.26% <100.00%> (+0.25%)	⬆️
cirq-core/cirq/circuits/frozen_circuit_test.py	100.00% <100.00%> (ø)
...q-core/cirq/sim/density_matrix_simulation_state.py	99.02% <100.00%> (+0.10%)	⬆️
cirq-core/cirq/sim/simulation_state_test.py	98.51% <100.00%> (ø)
cirq-google/cirq_google/api/v2/sweeps_test.py	100.00% <100.00%> (ø)
cirq-google/cirq_google/cloud/quantum/__init__.py	100.00% <100.00%> (ø)
... and 4 more

... and 1 file with indirect coverage changes

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[dstrain115]

This looks pretty awkward to use. Since you usually get a FrozenCircuit from circuit.freeze(), should we add circuit.freeze(tags=tags) or a cirq.freeze_with_tags(tags)?