Add TaprootSigner variant to SignerProvider.

Add TaprootSigner variant to SignerProvider.

Previously, SignerProvider was not laid out to support multiple signer
types. However, with the distinction between ECDSA and Taproot signers,
we now need to account for SignerProviders needing to support both.

This approach does mean that if ever we introduced another signer type
in the future, all implementers of SignerProvider would need to add it
as an associated type, and would also need to write a set of dummy
implementations for any Signer trait they do not wish to support.

For the time being, the TaprootSigner associated type is cfg-gated.