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 #1854.

Closes #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

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but also to its interactions with existing and future code.

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Testing

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