Add string.format

[DangerOnTheRanger]

This PR adds a format receiver function to CEL strings. The syntax is similar to the printf implementations found in various languages. For example, "%s and a number: %d".format(["a string", 42]) evaluates to "a string and a number: 42".

[jpbetz]

Not a full review pass, but noticed a few things.

[TristonianJones]

@DangerOnTheRanger Please move this functionality to ext/strings.go since string formatting would be an extension function that's optionally enabled rather than a part of the CEL standard library.

[TristonianJones]

@DangerOnTheRanger I have some questions based on what I've seen. Can you tell me the intended behavior for the following calls:

// error based on list literal size or simply a reuse of the first argument in the string?
"{0} < {0}".format([1, 2]) 

// How are brackets escaped?
"{{0}: {1}}".format([key, value])

It sounds like you and Joe have adequately answered whether the digits are Ascii-only, and I'm fine with that as well.

[DangerOnTheRanger]

Kermit Alexander II I have some questions based on what I've seen. Can you tell me the intended behavior for the following calls:

// error based on list literal size or simply a reuse of the first argument in the string?
"{0} < {0}".format([1, 2]) 

// How are brackets escaped?
"{{0}: {1}}".format([key, value])

It sounds like you and Joe have adequately answered whether the digits are Ascii-only, and I'm fine with that as well.

For the first one, as currently written the implementation would accept this (the first argument would be reused) but after offline discussion with @jpbetz I think this should result in a compile-time error if the formatting string is a constant.

For the second, that would result in an error ({{ is treated as an escape sequence so the first } is unmatched), which feels reasonable to me. Admittedly the "correct form" of the formatting string ({{{0}: {1}}}) does look very verbose, but for use cases where the user wants to use the same characters that are used for formatting, (like { and } in this case) I'm not sure there's a way around that.

[jpbetz]

/gcbrun

LGTM

[DangerOnTheRanger]

(force push to fix merge conflict)

[TristonianJones]

/gcbrun

[TristonianJones]

/gcbrun

[TristonianJones]

/gcbrun

[cici37]

@jpbetz Would you mind checking if the request changes addressed? Looks like the change request is blocking the merging process on this one. Thank you :)