Update method signature support to adhere to spec

[marcoferrer]

The current implementation of support for method signature options is compliant with the specification defined in googleapis/api-common-protos/client.proto#L79

The following changes need to be made to become spec compliant.

• Each instance of a method signature annotation is to be treated as a target signature
• The fields to include in the signature are to be defined as a comma delimited string

The current implementation incorrectly assumes that there is only ever one target method signature to implement. It also collects all of the signature annotation entries on a method and treats them as the fields to output in the target signature.