all: Consider enforcing that all strings are UTF-8

[vangent]

Per https://github.com/google/go-cloud/blob/master/internal/docs/design.md#strings, Go CDK supports all UTF-8 strings, and drivers are required to do escaping if needed.

Currently, behavior for non-UTF-8 strings is undefined; some providers might error out, others might replace non-UTF-8 characters with unicode.ReplacementChar, etc.

Perhaps we should have the concrete type check for valid UTF-8 strings so we can consistently return an error.

[vangent]

Fixed with #1322.