signature verification refactoring

now there are two signature verification implementations:

• LocalSignatureVerifier - refactored version of the initial verification procedure, process did:web DIDs locally, then verifies signature with JsonWebSignature2020Suite
• UniSignatureVerifier - resolves DIDs with help of external uni-resolver, then verifies signature using one of: RsaSignature2018Suite, Ed25519Signature2018Suite, Ed25519Signature2020Suite, JcsEd25519Signature2020Suite, EcdsaSecp256k1Signature2019Suite, EcdsaKoblitzSignature2016Suite, JcsEcdsaSecp256k1Signature2019Suite, BbsBlsSignature2020Suite, JsonWebSignature2020Suite

the verifier implementation can be set in application properties.

also, some improvements around DocumentLoader added: context pre-loading, caching, configurability, etc..