feat: add support for v1

[busunkim96]

Primarily adds support for v1 API.

Also includes some template updates.

[busunkim96]

index.rst needs a small fix:

Traceback (most recent call last):
  File "/tmpfs/src/github/python-secret-manager/.nox/docs/lib/python3.7/site-packages/sphinx/cmd/build.py", line 276, in build_main
    app.build(args.force_all, filenames)
  File "/tmpfs/src/github/python-secret-manager/.nox/docs/lib/python3.7/site-packages/sphinx/application.py", line 349, in build
    self.builder.build_update()
  File "/tmpfs/src/github/python-secret-manager/.nox/docs/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 299, in build_update
    len(to_build))
  File "/tmpfs/src/github/python-secret-manager/.nox/docs/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 311, in build
    updated_docnames = set(self.read())
  File "/usr/local/lib/python3.7/contextlib.py", line 119, in __exit__
    next(self.gen)
  File "/tmpfs/src/github/python-secret-manager/.nox/docs/lib/python3.7/site-packages/sphinx/util/logging.py", line 219, in pending_warnings
    memhandler.flushTo(logger)
  File "/tmpfs/src/github/python-secret-manager/.nox/docs/lib/python3.7/site-packages/sphinx/util/logging.py", line 184, in flushTo
    logger.handle(record)
  File "/usr/local/lib/python3.7/logging/__init__.py", line 1524, in handle
    self.callHandlers(record)
  File "/usr/local/lib/python3.7/logging/__init__.py", line 1586, in callHandlers
    hdlr.handle(record)
  File "/usr/local/lib/python3.7/logging/__init__.py", line 890, in handle
    rv = self.filter(record)
  File "/usr/local/lib/python3.7/logging/__init__.py", line 751, in filter
    result = f.filter(record)
  File "/tmpfs/src/github/python-secret-manager/.nox/docs/lib/python3.7/site-packages/sphinx/util/logging.py", line 404, in filter
    raise SphinxWarning(location + ":" + message)
sphinx.errors.SphinxWarning: /tmpfs/src/github/python-secret-manager/docs/index.rst:4:Title underline too short.

v1 Api Reference
-------------

Warning, treated as error:
/tmpfs/src/github/python-secret-manager/docs/index.rst:4:Title underline too short.

[gcf-merge-on-green]

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