Update internal/cliext getters to return their full type

[mathispesch]

This PR is a proposed fix for #201. It updates the getters introduced in #194 to return their full type so that later parsing of the expected full types doesn't fail after urfave/cli calls .Get().

With this change, the flags are read correctly (tested with map --headers and enum value --informat):

❯ go build && ./grpc-client-cli -H "authorization: Bearer sometoken" --informat text --service grpc.health.v1.Health --method Check --input /tmp/testi --verbose localhost:8443
Error: invalid input text: proto: syntax error (line 1:1): invalid field name: {

Method: /grpc.health.v1.Health/Check
Status: 2 Unknown

Request Headers:
authorization: [Bearer sometoken]
user-agent: [grpc-go/1.79.1]

Request duration: 0s
Request size: 0 bytes
Response size: 0 bytes

[vadimi]

thanks @mathispesch, that makes sense, merging