TypeScript should support validating multiple signatures and filtering by label

At the moment, we only support validating one wba signature. We should provide support for more. The API is TBD. Rust uses a selection method.

Start by examining the TypeScript signature-validation API and the Rust selection method mentioned in the issue. The API for TypeScript is still TBD, so first clarify how multiple signatures should be selected and how filtering by label should work; done means supporting validation of multiple WBA signatures with label filtering.

Written by the indexing model from the issue text.