feat(functions): allow custom functions entrypoint path

[sweatybridge]

What kind of change does this PR introduce?

feature

What is the new behavior?

Allow functions entrypoint to be customized, ie. functions/my-slug/index.js

TODO:

• support arbitrary entrypoint path, ie. ../slug/sample.js

Additional context

Add any other context or screenshots.

[coveralls]

Pull Request Test Coverage Report for Build 11585965905

Details

• 8 of 10 (80.0%) changed or added relevant lines in 3 files are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.02%) to 59.801%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
internal/functions/serve/serve.go	2	4	50.0%

Files with Coverage Reduction	New Missed Lines	%
internal/gen/keys/keys.go	5	12.9%

Totals
Change from base Build 11576142195:	-0.02%
Covered Lines:	6361
Relevant Lines:	10637

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💛 - Coveralls