CLI - Implement new `login` flow

[bfops]

Description of Changes

Add the spacetime login command, which gets a login token from the web server.

This currently just implements the command and adds the returned session token into the config file. It does not use the token anywhere!

Getting the token requires https://github.com/clockworklabs/spacetime-web/pull/464. Using the token requires https://github.com/clockworklabs/SpacetimeDB/pull/1854.

Closes https://github.com/clockworklabs/SpacetimeDB/issues/1847.

API and ABI breaking changes

This breaks the current flow for using any CLI commands that interact with a server. They will now require logging in first.

Expected complexity level and risk

How complicated do you think these changes are? Grade on a scale from 1 to 5, where 1 is a trivial change, and 5 is a deep-reaching and complex change.

This complexity rating applies not only to the complexity apparent in the diff, but also to its interactions with existing and future code.

If you answered more than a 2, explain what is complex about the PR, and what other components it interacts with in potentially concerning ways.

Testing

Describe any testing you've done, and any testing you'd like your reviewers to do, so that you're confident that all the changes work as expected!

• [x] Write a test you've completed here.
• [ ] Write a test you want a reviewer to do here, so they can check it off when they're satisfied.

[bfops]

Combining this into https://github.com/clockworklabs/SpacetimeDB/pull/1878