[pkg/ottl] Add StringGetter

[TylerHelmuth]

Adds a new concept to Getters that allow specifying type. These type-specific Getters can be used by functions that require a specific type instead of using the normal Getter that always returns an interface. This PR implements a StringGetter, but we can add Getter's for int64, float64, bool, pcommon.Map, etc in the future.

With these Getters functions and rely on OTTL for type assertion and with the ongoing error handling discussion these will help standardize how functions handle and return errors.

Related to #16519

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[evan-bradley]

I like the approach of solving this using Getters. I think having a StringLikeGetter or similar for coercing non-string values to strings could potentially help with cases like IsMatch in the future.

[TylerHelmuth]

I think having a StringLikeGetter or similar for coercing non-string values to strings could potentially help with cases like IsMatch in the future.

This is an interesting idea. I like it more than forcing functions that can handle multiple types to using the basic Getter and then do type assertion themselves.