Remove username prompt on buf registry login

[emcfarlane]

This removes the requirement to provide your username when calling buf registry login. On login the token is validated by fetching the current user. The username is used only to validate that the current token is associated with the user but is otherwise redundant. The flag has now been marked deprecated and is hidden.

The output of the login command is also updated to be more user friendly prompting the user to visit the web UI to create a token if they do not have one. On successful login the user is informed that they are logged in as the current user.

Token via prompt:

$ buf registry login
Enter the BSR token created at https://buf.build/settings/user.

Token:
Logged in as emcfarlane. Credentials saved to /Users/edward/.netrc.

Token via stdin:

$ echo ${BUF_TOKEN} | buf registry login buf.build --token-stdin
Logged in as emcfarlane. Credentials saved to /Users/edward/.netrc.

[github-actions]

The latest Buf updates on your PR.

Name	Status
build	✅ passed
lint	✅ passed
format	✅ passed
breaking	✅ passed

[doriable]

This makes a lot of sense to me. I like that --username continues to be available -- I mulled over whether we should consider marking the flag as deprecated, but I think the fact that it continues to provide validation makes a lot of sense.

[doriable]

We should probably add a changelog entry to this.