feat: add iam methods to templates

[busunkim96]

Closes #536

This adds a new generator option add-iam-methods.

The option adds the three standard IAM methods get_iam_policy, set_iam_policy, and test_iam_permissions. This option is a replacement for reroute_to_grpc_interface in the monolith. reroute_to_grpc_interface is used by KMS and Pub/Sub to expose the IAM methods. Future clients are not expected to use this option.

This PR copy-pastes the IAM methods from the cloud functions client into the templates. The only edits are to the endpoint.

I tried get_iam_policy with KMS and I get the same result as the existing library.

The down-side of copy-pasting is that this code is prone to being missed in changes to the templated methods/tests. I tried adding the methods at the API schema level but was unsuccessful. I'm not very familiar with how the schema works, so if there's a good way to make the change at that level I'm happy to rework this PR.

Failing showcase tests should pass once proto-plus 1.3.2 makes it to PyPI.

[codecov]

Codecov Report

Merging #545 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #545   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           26        26           
  Lines         1507      1477   -30     
  Branches       308       308           
=========================================
- Hits          1507      1477   -30     

Impacted Files	Coverage Δ
gapic/generator/options.py	100.00% <100.00%> (ø)
gapic/schema/api.py	100.00% <0.00%> (ø)
gapic/schema/imp.py	100.00% <0.00%> (ø)
gapic/utils/cache.py	100.00% <0.00%> (ø)
gapic/schema/metadata.py	100.00% <0.00%> (ø)
gapic/schema/wrappers.py	100.00% <0.00%> (ø)
gapic/samplegen/samplegen.py	100.00% <0.00%> (ø)
gapic/samplegen_utils/types.py	100.00% <0.00%> (ø)

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