Add new CLI auth flow w/ tests and docs

[joshmeek]

Thanks for contributing to Prefect!

Please describe your work and make sure your PR:

• adds new tests (if appropriate)
• updates CHANGELOG.md (if appropriate)
• updates docstrings for any new functions or function arguments, including docs/outline.toml for API reference docs (if appropriate)

Note that your PR will not be reviewed unless all three boxes are checked.

What does this PR change?

Closes #1417
Adds new CLI commands for working with Prefect Cloud auth
Related to changes in #1423

e.g.

        $ prefect auth login --token MY_TOKEN
        Log in successful

        $ prefect auth list-tenants
        NAME                        SLUG                        ID
        Test Person                 test-person                 816sghf2-4d51-4338-a333-1771gns7614d
        test@prefect.io's Account   test-prefect-io-s-account   1971hs9f-e8ha-4a33-8c33-64512gds86g1 

        $ prefect auth switch-tenants --slug test-person
        Tenant switched

        $ prefect auth list-tenants
        NAME                        SLUG                        ID
        Test Person                 test-person                 816sghf2-4d51-4338-a333-1771gns7614d    *
        test@prefect.io's Account   test-prefect-io-s-account   1971hs9f-e8ha-4a33-8c33-64512gds86g1 

        $ prefect auth logout
        Logged out from tenant 

        $ prefect auth list-tenants
        NAME                        SLUG                        ID
        Test Person                 test-person                 816sghf2-4d51-4338-a333-1771gns7614d
        test@prefect.io's Account   test-prefect-io-s-account   1971hs9f-e8ha-4a33-8c33-64512gds86g1 

Depends on #1426

Why is this PR important?

Makes auth really easy to work with in the CLI

[codecov]

Codecov Report

Merging #1431 into master will increase coverage by 0.03%.
The diff coverage is 95.94%.

[cicdw]

Minor nits and picks - this looks super useful

[joshmeek]

Yeah @cicdw I already love it because it removes the need for multiple manually changing tenant-scoped tokens in my config.toml

[jlowin]

Quick hits:

• login should automatically use a default membership (if available) to switch tenants off the bat
• disable all auth commands if a token is found in config (because it'll be given preference)
• commands for listing tokens (name, id)
• commands for revoking tokens (by id)

[joshmeek]

@jlowin @cicdw Updated as per comments