Add support for functions

[tchaton]

What does this PR do?

This PR optimize the execution of functions provided to configure_commands or configure_api by executing them direclty within the Rest API process.

Fixes #<issue_number>

Does your PR introduce any breaking changes? If yes, please list them.

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Did you have fun?

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cc @Borda

[manskx]

Looks good, I left a few comments.

[lantiga]

LGTM