Allow generic events of of content type json only

[MosesMuiru]

• fixes: Generic Events Webhook: Accept empty POST request(s) body #753

Reason for error occurrence:
The API is not designed to handle any other content-type other than application/json. Hence the the server crashes.

Solution:
filter the content type and give a more descriptive error than (500). To 415

[github-actions]

📦 To trigger a firmware build, comment /build below.
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Valid target names:

• ex_nvr_rpi4
• ex_nvr_rpi5
• giraffe

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Examples:
/build version=1.0.0-test
/build ex_nvr_rpi5 version=1.0.0-test.123

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