Support HMAC Validation for Filebeat http_endpoint input

[SpencerLN]

Describe the enhancement:
Add configuration options to enable the use of HMAC to validate requests received to the http_endpoint input. This configuration should be flexible and allow the use of various HMAC hash types, keys, key prefixes, etc. The user should also be able to configure where the signature comes from (i.e. which Header).

Describe a specific use case for the enhancement or feature:

Many services (GitHub, Dropbox, etc.) suggest authenticating their webhook requests using HMAC.

https://www.dropbox.com/developers/reference/webhooks
https://docs.github.com/en/developers/webhooks-and-events/securing-your-webhooks

Currently, there is no way to authenticate that the webhook you receive is actually sent by services that do not support using Basic authentication or an arbitrary header value. Adding this feature would allow users to securely consume data from additional services.

[elasticmachine]

Pinging @elastic/agent (Team:Agent)

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)