Create HTTPX Custom Auth to handle token refresh

[ericcccsliu]

After creating a function to get the currently cached Enode access token and another to refresh it if necessary, we'll want to create a custom HTTPx Auth class to handle the logic for actually determining whether our current token needs to be refreshed. The way we do this is by, after someone makes a request to Enode's API, checking the response HTTP status code. If it's a 401, then we know our access token is no longer valid, and we must request another.

The way this is implemented in HTTPx is by creating a custom Auth class, which we'll pass into our HTTPx clients auth argument. See the example under this paragraph in the docs.

• Create the custom Auth class to handle automatically determining token expiration and refreshing it, then re-requesting the resource

Here's how this code looks in TypeScript in our own mock API:

https://github.com/openclimatefix/pv-sites-mobile/blob/c5f47fde4c6c2cce6f145f58edbeddbca687be11/lib/enode.ts#L85-L108