feat: add get-token command for fetching user-facing tokens for an API

[paddycarey]

https://auth0team.atlassian.net/browse/A0CLI-18

This PR adds the ability to fetch a token for a given client and API, with user-specified scopes.

NOTE: This is the flow for a user-facing token, the client credentials flow is being tackled seperately.

This could 100% be refactored, will tackle that after.

[jfatta]

I'd suggest having this under auth0 clients get-token for consistency. But if they're waiting for this for the demo feel free to merge it

[paddycarey]

To me it feels more like a top-level command like try-login, since it can be used without specifying a client and actually relates to APIs too.

I think we should keep subcommands for actions related to the top-level resource, like CRUD operations on clients/apis/rules/whatever. But I think that's a higher-level discussion for another forum 🤷🏻

[jfatta]

makes sense