Expose AddFunction API for CESQL Parser

[Cali0707]

As discussed in #1045 , it would be good to provide end users of the SDK the ability to add custom functions to the CESQL parser and runtime.

I think this would probably start with changes in https://github.com/cloudevents/sdk-go/blob/f97061a7e72b17f63120aa553123edb28c49e490/sql/v2/runtime/functions_resolver.go#L23, but it will likely need changes in other areas as well

[Cali0707]

@duglin I believe @dgeorgievski has offered to contribute a PR for this

[dgeorgievski]

Just for the record I am willing to work on this PR

[duglin]

thanks @dgeorgievski