[tsp-client] Improve generate-config-files command

Try to simplify the command so it doesn't always require passing in a path to an emitter's package.json.

Some ideas:
- Save the github url to the emitter's package.json in a tsp-client config file (the metadata file)
- Also just generally allow passing in a github url to the --package-json flag for parsing.
- Save ONLY the url to the metadata file automatically when it's passed in to the --package-json flag. Don't save a local path to the metadata file.
- Try pulling dependency info from npm directly. Can we get our custom package.json fields like "azure-sdk/emitter-package-json-pinning"?

Additional considerations, overrides would have to be passed for every command execution since these can and probably do change for every run.