Added CEL to list of valid ExpressionLangs

[venera-program]

What this PR does / why we need it:
According to the 0.8 spec, we shouldn't be verifying against a list of "accepted" ExpressionLangs, only that jq is the default one.

Serverless Workflow does not mandate the use of jq and it's possible to use an expression language of your choice with the restriction that a single one must be used for all expressions in a workflow definition. If a different expression language needs to be used, make sure to set the workflow expressionLang property to identify it to runtime implementations.

CEL is a commonly used expression language which sees use in implementations of this sdk-go library. Support for it as a valid ExpressionLang was removed in or around 4afc5f3.

Additional information (if needed):
Optimally, we would want the validation to be more dynamic than adding another language to the list (as I'm doing in this PR), but I do not know enough about the validation processes to present a robust enough solution for that.

[spolti]

That's fine by me.
@ricardozanini ?

[ricardozanini]

Optimally, we would want the validation to be more dynamic than adding another language to the list (as I'm doing in this PR), but I do not know enough about the validation processes to present a robust enough solution for that.

@venera-program I agree. Ideally, we should remove the enum and accept any string, defaulting to jq.

[venera-program]

@spolti
Could I get a merge if everything is approved?

[spolti]

@venera-program do you need a new release?

[venera-program]

@spolti
I'm vendoring from commit atm so it doesn't matter for me either way, up to you if a release is necessary.