fix(serving-functions): wait for watcher to be ready after registering a new function

[erezrokah]

- Summary

Enables the functions serving tests and fixes the issue that caused them to fail.
Since we didn't wait for buildFunctionAndWatchFiles to finish when running registerFunction, the following could happen:

1. We start the initial scan and discover a new function hello.ts
2. We register it and start building it
3. The user changes hello.ts (triggering a change event)
4. We finish building the function and setup the watcher for hello.ts

As a result the change event is skipped.

The current solution is to wait for buildFunctionAndWatchFiles to complete as a part of registerFunction which has a performance cost of having to wait for all functions to build before starting the functions server.
I couldn't think of another way to fix it, happy to get some more feedback on this.

- Test plan

Existing tests

- A picture of a cute animal (not mandatory but encouraged)
🐒

[github-actions]

📊 Benchmark results

Comparing with 1c38b9c

Package size: 330 MB

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Legend

• T-30 (1c38b9c): 330 MB
• T-29 (8b3a584): 330 MB
• T-28 (992461d): 330 MB
• T-27 (728826e): 330 MB
• T-26 (321e2b2): 330 MB
• T-25 (e459351): 330 MB
• T-24 (320d42f): 330 MB
• T-23 (b7f84e4): 330 MB
• T-22 (20a9de9): 330 MB
• T-21 (315d616): 330 MB
• T-20 (aa6c8fb): 330 MB
• T-19 (9ef19aa): 330 MB
• T-18 (75299ba): 330 MB
• T-17 (32c6086): 330 MB
• T-16 (ced0c93): 330 MB
• T-15 (7ccf859): 330 MB
• T-14 (278557a): 330 MB
• T-13 (db8c431): 330 MB
• T-12 (a53b36f): 330 MB
• T-11 (08d87ff): 329 MB
• T-10 (0767251): 329 MB
• T-9 (307e128): 330 MB
• T-8 (c68efb2): 330 MB
• T-7 (bbb78bd): 330 MB
• T-6 (2c6a57a): 330 MB
• T-5 (8bfdb2f): 330 MB
• T-4 (2e18003): 330 MB
• T-3 (fd3d2f7): 330 MB
• T-2 (d451cb3): 330 MB
• T-1 (c381072): 330 MB
• T (current commit): 330 MB

[erezrokah]

Adding more context - we don't know which files to track until we finish building the functions for the first time.

As we don't want to wait for all functions to build before starting this function server closing in favor of #2933 since it's not very likely that a user will manage to change a file before the first build.

We might find a way to improve this by tracking file changes on the directory before the first build and accounting for those, but that won't work for supporting files that are outside the functions directory.