[SDK-1577] [SDK-1578] Add login functionality

[adamjmcgrath]

Description

Add login functionality to the auth0 hook

Testing

• This change adds test coverage for new/changed/fixed functionality

Checklist

• All active GitHub checks for tests, formatting, and security are passing
• The correct base branch is being used, if not master

[adamjmcgrath]

I wanted to use manual mocks to mock the Auth0Client class https://jestjs.io/docs/en/es6-class-mocks#manual-mock

This is so that I can export the methods as individual mocked methods, so I can do:

expect(getTokenSilently).toHaveBeenCalled()

Rather than use automatic mocks and do this everywhere:

expect((Auth0Client as any as jest.Mock).mock.instances[0].getTokenSilently).toHaveBeenCalled()

Since TS doesn't understand mocking in Jest, we do have to ts-ignore the additional imports

[stevehobbsdev]

Not something to change in this PR, but I think we need to think about why we're mocking the SPA SDK and whether that's the right thing, instead of mocking the resulting HTTP calls (or handling them with some kind of test proxy).

For the same reason we don't test or mock private methods, the fact we're using the SDK internally should just be that - an internal detail. HTTP calls out to the auth service are where the 'seams' are and it's the seams that should ideally be mocked. If we go down the path of mocking dependencies like these, we can lose sight of what happens when those dependencies change if we're always mocking out the ideal scenario.