Fully test and fix types in StringFormat implementation #257
Description
Our current StringFormat implementation is balking at the cel-python types we're using. Additionally, the protovalidate-conformance suite only really tests the %s verb, so we ought to have tests over the rest of the verbs.
The cel-spec test suite for formatting can be found here: https://github.com/google/cel-spec/blob/master/tests/simple/testdata/string_ext.textproto. Here is cel-go's explanation of how format should work.
Additionally, while we currently can add these tests in-repo, it'd behoove us to contribute a conformant implementation upstream to cel-python.