Reduce code duplication in method signatures throughout the code base

[parthea]

See the diff in PR https://github.com/googleapis/gapic-generator-python/pull/2253/files which shows changes to the following code which appears throughout the code base

        retry: OptionalRetry = gapic_v1.method.DEFAULT,
        timeout: Union[float, object] = gapic_v1.method.DEFAULT,
        metadata: Sequence[Tuple[str, str]] = (),
    )

We can reduce code duplication by using common code for a common method signature. As a pre-requisite, we need to fix the following issues

Address the inconsistency with the default values in timeout.

See here

gapic-generator-python/gapic/templates/%namespace/%name_%version/%sub/services/%service/_client_macros.j2

Lines 31 to 32 in 515dc8a

	retry: OptionalRetry = gapic_v1.method.DEFAULT,
	timeout: Union[float, object] = gapic_v1.method.DEFAULT,

and here

gapic-generator-python/gapic/templates/%namespace/%name_%version/%sub/services/%service/transports/rest.py.j2

Line 205 in 515dc8a

timeout: Optional[float]=None,

Address the inconsistency in the type for Async REST and Async Mixins and pagers, where we have bothOptionalAsyncRetry and OptionalRetry defined as the type for retry

See here

gapic-generator-python/gapic/templates/%namespace/%name_%version/%sub/services/%service/transports/rest_asyncio.py.j2

Line 62 in 515dc8a

OptionalRetry = Union[retries.AsyncRetry, gapic_v1.method._MethodDefault, None]

and here

gapic-generator-python/gapic/templates/%namespace/%name_%version/%sub/services/%service/_async_mixins.py.j2

Line 10 in 515dc8a

retry: OptionalRetry = gapic_v1.method.DEFAULT,

and here
OptionalAsyncRetry

gapic-generator-python/gapic/templates/%namespace/%name_%version/%sub/services/%service/pagers.py.j2

Line 16 in 515dc8a

OptionalAsyncRetry = Union[retries_async.AsyncRetry, gapic_v1.method._MethodDefault, None]