Move validation methods to public api

[jimshowalter]

As discussed in #2935, we need to add our own validation.

In addition, we need to pre-check the values coming in from headers, to avoid trying to call BaggageBuilder.put with a null or empty string.

Currently the code to validate keys and values is in private static methods in ImmutableBuilder, and those in turn call methods that are in io.opentelemetry.api.internal.StringUtils, which is an internal package.

We don't want to just copy this code.

(If it's safe to call code in internal packages, let us know and that part we'll drop from this request.)