Add ovsx login equivalent to vsce login

[sparr]

It would be nice to be able to supply an access token once and have it saved for future publish operations, instead of needing to provide it via --pat with every operation.

The vsce implementation uses interactive prompts and can remember tokens for multiple publishers, but a command with explicit parameters and/or storing a token for just a single publisher would also be viable in a simpler solution here.

[amvanbaren]

Or just use the vsce implementation, similar to how ovsx uses vsce's package command.