Added "hook" for server start event

[wcarmon]

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This allows tests to detect when server starts. Tests will execute faster than polling. Avoids exposing the server instance. Should have zero impact on existing usages

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[google-cla[bot]]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

:memo: Please visit https://cla.developers.google.com/ to sign.

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[wcarmon]

@googlebot I signed it!

[grant]

Sorry @wcarmon, features like this need to be discussed much further before a PR is sent.

Do you mind creating a GitHub issue first and discussing your requirements?

[josephlewis42]

Thanks for the PR! Since there hasn't been discussion about this for a few years I'm going to close it out for now.

It looks like this is related to a general-purpose lifecycle hooks proposal: https://github.com/GoogleCloudPlatform/functions-framework/issues/56 It's likely we'd want to approve that before taking any code for specific functions frameworks so we can ensure they all behave the same way.