feat: Matrix SSO Callback Implementation

[aturkenov]

Last matrix specification includes SSO Authentication using custom identity providers.
That feature was tested on Authentik.

Issue:
Im ran custom web based fluffychat instance.
I wanna use matrix sso flow (example https://matrix.example.com/_matrix/client/v3/login/sso/redirect/oidc-my-custom?redirectUrl=https://chat.example.com/). After successful authentication client should be redirected to fluffychat with temporary loginToken, then fluffychat should parse that token from the URL and obtain permanent access token.
Its required for skipping homeserver picking step.

• Code formatting and import sorting has been done with dart format lib/ test/ and dart run import_sorter:main --no-comments
• The commit message uses the format of Conventional Commits
• The commit message describes what has been changed, why it has been changed and how it has been changed
• Every new feature or change of the design/GUI is linked to an approved design proposal in an issue
• Every new feature in the app or the build system has a strategy how this will be tested and maintained from now on for every release, e.g. a volunteer who takes over maintainership

Pull Request has been tested on:

• Android
• iOS
• Browser (Chromium based)
• Browser (Firefox based)
• Browser (WebKit based)
• Desktop Linux
• Desktop Windows
• Desktop macOS

Thank you @krille-chan!

[krille-chan]

Matrix SSO flow is already implemented. Please explain why we need this.