Block UI elements to handle authorization

[Pwuts]

• Part of Implement front-to-back GitHub OAuth support on blocks #8002

This ticket entails implementing the UI for both picking and initiating creation of new credentials.

For the block input component that handles the credentials, we'll need to add a new custom type of input. We can include info in the JSON schema like what kinds of credentials the provider supports, so that the block can offer an appropriate UI to enter e.g. either an API key or click a button to log in.

Currently supported types (source):

• OAuth
• API key

If no credentials for a given provider are configured yet, the credential input should present a button which:

• if the provider supports multiple auth methods -> opens a modal where the user can click a login button or enter and save their API key
• if the provider supports only OAuth -> directly initiates the log-in flow (Integration OAuth front end mechanism #8004)
• if the provider supports only an API key -> opens a modal where a user can enter and save their API key

When an OAuth flow is completed successfully, the modal should close.

If credentials for a given provider are already available, the user should be offered a drop-down with options:

• [Available Credentials 1]
• [Available Credentials 2…n]
• [+ Add account]

Clicking [+ Add account] will behave the same as the auth creation/initiation button described above.