feat(builders): add send_headers setter to SapiBuilder

[Qard]

Description

I need send_headers (plural) rather than just send_header (singular) to be able to know when sending of all headers completes. I'm using this to support streaming so I can return a response body stream as soon as headers are available and then I can read from it while the request continues to be processed in another thread.

Checklist

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• I have read the contribution guidelines.
• I have added tests that prove my code works as expected.
• I have added documentation if applicable.
• I have added a migration guide if applicable.

❤️ Thank you for your contribution!

[coveralls]

Pull Request Test Coverage Report for Build 18922539846

Details

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.05%) to 31.084%

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Totals
Change from base Build 18909521820:	0.05%
Covered Lines:	1359
Relevant Lines:	4372

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💛 - Coveralls

[ptondereau]

Seems good enough, thanks!