Update Cloud Client for new USER-scoped API tokens

[jlowin]

This PR supersedes #1410

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• 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)

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What does this PR change?

This PR adds a new auth flow to the Python Cloud client, without changing its existing behavior. However, it does rename some private attributes.

The primary contribution of this PR is to support USER-scoped Cloud API tokens, which function as personal access tokens. USER-scoped tokens can not be used to interact with the Cloud API in a significant way, but are able to authenticate with the API and receive short-lived JWTs in return. These JWTs are full auth tokens representing a user's membership in a specific Cloud tenant.

The user flow works like this:

1. User instantiates a Cloud client and passes it a USER-scoped API token
2. User calls client.save_api_token() to write the token to local storage so it persists across Python sessions.
3. User calls client.get_available_tenants() to see what tenants they can login to
4. User calls client.log_to_tenant(tenant_id=<>, tenant_slug=<>) (with one or the other argument). When this happens, an ephemeral JWT is received as well as a refresh token. Note this step is also persisted across sessions.
5. The User can now make API calls as if they were logged in to the tenant in question, for example client.graphql(...). The Client is managing all refreshing and exchanging of JWTs in the background.
6. The User is finished, so they call client.logout_from_tenant(). This discards the JWT access token.

The TOKEN- and AGENT- scoped flows are unchanged:

1. Client instantiated with an API token (or receives one via env var/config)
2. Client includes the API token in all calls.

[codecov]

Codecov Report

Merging #1423 into master will increase coverage by 0.03%.
The diff coverage is 98.87%.

[joshmeek]

RUNNER

[jlowin]

TY