OIDC: navigate to authorization endpoint

[kerryarchibald]

Fixes element-hq/element-web#25574
With matrix-org/matrix-js-sdk#3499

3.1.1. Authorization Code Flow Steps

1. Client prepares an Authentication Request containing the desired request parameters.
2. Client sends the request to the Authorization Server.
3. Authorization Server Authenticates the End-User.
4. Authorization Server obtains End-User Consent/Authorization.
5. Authorization Server sends the End-User back to the Client with an Authorization Code.
6. Client requests a response using the Authorization Code at the Token Endpoint.
7. Client receives a response that contains an ID Token and Access Token in the response body.
8. Client validates the ID token and retrieves the End-User's Subject Identifier.

This task addresses steps 1-5.

Splitting this flow into two parts to keep change set a reasonable size.

• adds UI for oidcNativeFlow (Continue button like existing oidc-aware flow)
• generates authorization params and navigates to authorization endpoint

Checklist

• Tests written for new code (and old code if feasible)
• Linter and other CI checks pass
• Sign-off given on the changes (see CONTRIBUTING.md)

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Here's what your changelog entry will look like:

✨ Features

• OIDC: navigate to authorization endpoint (#11096). Fixes OIDC: Make authorization request element-hq/element-web#25574. Contributed by @kerryarchibald.

[t3chguy]

Looks sane