App: Refresh Tokens Implementation

[langleyd]

Resolves #5292

Dependant on matrix-org/matrix-ios-sdk#1319

Included in this PR

• Introduces BuildSettings.authEnableRefreshTokens which enables refresh token auth on the client, which takes effect on the next user login. I the user is already logged in access token auth will remain in use until the user logs out.
• Adds refresh token and the current access token expiration timestamp to the MXKAccount coding so that it is persisted when accounts are saved.
• We also listen for the MXRestClientDidRefreshTokensNotification and save accounts when the tokens change.
• Split out the a new MXKAccountData class from MXKAccount that contains just the codable data so that we can read/write account data to disk without the sideeffects that occur with in MXKAccount.
• Use MXKAccountData to read credentials from disk and write them back in a single transaction using NSFileCoordinator. This is used within the refresh token flow to avoid data race conditions/corruption when refreshing tokens across processes (App + Notification, Sharing, Siri extensions).
• Fixed a bug of the grey spinner staying up on the HomeViewControllerWithBannerWrapperViewController. It now passes start/stop events through to the underlying HomeViewController which correctly listens to sync events via inheriting MXKViewController.

Todo:

• Further testing on extension use cases.
• Add posthog events for unauthenticated.
• Test disconnecting and reconnecting to an identity server from settings.

[langleyd]

Just realised refresh tokens will be a bit of a problem with the notification extension. The refresh token rotates each time you request a new access token (so you get a new access token and refresh token in the response). So you need to coordinate any processes and insure there is only 1 refresh exchange happening at any point in time. If you request a refresh with an invalid refresh I think you'l be signed out.

[github-actions]

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