Remove the need to use the getKeyDidMappingByVerificationMethod method

[TimoGlastra]

Currently you have to use the getKeyDidMappingByVerificationMethod method to get access to the getKeyFromVerificationMethod method for a specific key type.

However this is a bit verbose and doesn't work nicely for the api.

E.g currently we have to write this:

    const didsApi = agentContext.dependencyManager.resolve(DidsApi)
    const didResult = await didsApi.resolve(`did:sov:${issuerId}`)

    if (!didResult.didDocument) throw new AriesFrameworkError('Unable to resolve did')

    const verificationMethod = didResult.didDocument.dereferenceKey('did:sov:${issuerId}#key-1')
    const { getKeyFromVerificationMethod } = getKeyDidMappingByVerificationMethod(verificationMethod)
    const key = getKeyFromVerificationMethod(verificationMethod)

it would be nice if we can just do this:

    const didsApi = agentContext.dependencyManager.resolve(DidsApi)
    const didResult = await didsApi.resolve(`did:sov:${issuerId}`)

    if (!didResult.didDocument) throw new AriesFrameworkError('Unable to resolve did')

    const verificationMethod = didResult.didDocument.dereferenceKey('did:sov:${issuerId}#key-1')
   const key = getKeyFromVerificationMethod(verificationMethod)

So the getKeyFromVerificationMethod just calls getKeyDidMappingByVerificationMethod internally.