Add type annotations to Dogstatsd.timed function and TimedContextManagerDecorator class

[soceanainn]

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What does this PR do?

Fixes #797 by adding some types to Dogstatsd.timed() function.

Description of the Change

1. Provides some types to Dogstatsd.timed() function and TimedContextDecoratorManager.
2. This does not fully solve the issue with types not being preserved when using timed as a decorator, but typing that correctly requires features (typing.TypeVar, typing.ParamSpec and typing.overload) that are not available in earlier versions of Python supported by this library.

Alternate Designs

N/A

Possible Drawbacks

None.

Verification Process

Ran tox -e mypy and ensured it passed.

Additional Notes

Release Notes

Review checklist (to be filled by reviewers)

• Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have one changelog/ label attached. If applicable it should have the backward-incompatible label attached.
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