feat: add flag for gapic metadata

[software-dov]

As per the design doc, gapic metadata should not be generated by
default, and should be gated by the 'metadata' flag.

When invoking the generator via protoc, toggle the metadata flag like so:
--python_gapic_opt="metadata"

Subsequent Bazel integration is WIP.

[codecov]

Codecov Report

Merging #795 (b56416b) into master (7c185e8) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #795   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           26        27    +1     
  Lines         1608      1649   +41     
  Branches       328       337    +9     
=========================================
+ Hits          1608      1649   +41     

Impacted Files	Coverage Δ
gapic/utils/case.py	100.00% <ø> (ø)
gapic/utils/reserved_names.py	100.00% <ø> (ø)
gapic/generator/generator.py	100.00% <100.00%> (ø)
gapic/schema/api.py	100.00% <100.00%> (ø)
gapic/schema/wrappers.py	100.00% <100.00%> (ø)
gapic/utils/__init__.py	100.00% <100.00%> (ø)
gapic/utils/checks.py	100.00% <100.00%> (ø)
gapic/utils/options.py	100.00% <100.00%> (ø)

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[vchudnov-g]

Looks good. Thanks for doing this!

Before turning on the metadata file, we should resolve the issue in the other pr about the dual transports.