Support alternative http bindings in the gapic schema and adds support for parsing multiple bindings for one method.

Co-authored-by: Kenneth Bandes <kbandes@google.com>

* fix: improper types in pagers generation

Generators are Iterators not Iterables https://docs.python.org/3/library/typing.html#typing.Generator

* chore: update goldens

Co-authored-by: Bu Sun Kim <busunkim@google.com>
Co-authored-by: Bu Sun Kim <8822365+busunkim96@users.noreply.github.com>

🤖 I have created a release \*beep\* \*boop\*
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## [0.52.0](https://www.github.com/googleapis/gapic-generator-python/compare/v0.51.2...v0.52.0) (2021-09-29)


### Features

* Support alternative http bindings in the gapic schema. ([#993](https://www.github.com/googleapis/gapic-generator-python/issues/993)) ([041a726](https://www.github.com/googleapis/gapic-generator-python/commit/041a726b818cd67812d689c23757f31ec9964d66))


### Bug Fixes

* improper types in pagers generation ([#970](https://www.github.com/googleapis/gapic-generator-python/issues/970)) ([bba3eea](https://www.github.com/googleapis/gapic-generator-python/commit/bba3eea5d45fe57c0395ceef30402ad7880013d7))
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