x-pack/filebeat/input/cel: update function signature to new cel-go API

[efd6]

The current mito extention function definitions make use of the old definition mechanism which is complex and has some issues with compile-time type evaluation resulting in rejection of some valid programs. These issues can be fixed by moving to the new function definition API that came with cel-go v0.18.

A change is pending in the mito repo to make this shift. This will need to be integrated into beats and due to the significant change to the extension library, tested against a variety of integrations to ensure that the change does not introduce any regression.

[elasticmachine]

Pinging @elastic/security-service-integrations (Team:Security-Service Integrations)