fix: fix get_field for reserved names in field path

[busunkim96]

Texttospeech has a google.api.method_signature with the reserved name input. Append _ to field_paths in get_field so lookup succeeds.

  File "/usr/local/google/home/busunkim/.cache/bazel/_bazel_busunkim/d0728a13f0c84f7fc8315b82038f5e65/sandbox/linux-sandbox/1137/execroot/com_google_googleapis/bazel-out/host/bin/external/gapic_generator_python/gapic_plugin.runfiles/gapic_generator_python/gapic/schema/wrappers.py", line 819, in <genexpr>
    for name_and_field in filter_fields(sig)
  File "/usr/local/google/home/busunkim/.cache/bazel/_bazel_busunkim/d0728a13f0c84f7fc8315b82038f5e65/sandbox/linux-sandbox/1137/execroot/com_google_googleapis/bazel-out/host/bin/external/gapic_generator_python/gapic_plugin.runfiles/gapic_generator_python/gapic/schema/wrappers.py", line 807, in filter_fields
    field = self.input.get_field(*name.split('.'))
  File "/usr/local/google/home/busunkim/.cache/bazel/_bazel_busunkim/d0728a13f0c84f7fc8315b82038f5e65/sandbox/linux-sandbox/1137/execroot/com_google_googleapis/bazel-out/host/bin/external/gapic_generator_python/gapic_plugin.runfiles/gapic_generator_python/gapic/schema/wrappers.py", line 414, in get_field
    cursor = self.fields[field_path[0]]
KeyError: 'input'
--python_gapic_out: protoc-gen-python_gapic: Plugin failed with status code 1.
Target //google/cloud/texttospeech/v1beta1:texttospeech-v1beta1-py failed to build

https://github.com/googleapis/googleapis/blob/69697504d9eba1d064820c3085b4750767be6d08/google/cloud/texttospeech/v1beta1/cloud_tts.proto#L52

[codecov]

Codecov Report

Merging #722 (90d54ac) into master (f646767) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #722   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           26        26           
  Lines         1608      1609    +1     
  Branches       328       329    +1     
=========================================
+ Hits          1608      1609    +1     

Impacted Files	Coverage Δ
gapic/schema/wrappers.py	100.00% <100.00%> (ø)

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[busunkim96]

Closing this as I see @yon-mg addressed this in #716.