OIDC: Validate m.authentication configuration

[kerryarchibald]

For element-hq/element-web#25466

Validates .well-known discovery result to check

• m.authentication config is valid
• m.authentication.issuer has valid configuration

Adds m.authentication section to clientConfig

Details in issue

Result is not used in react-sdk yet, but can be tested by entering a server name in the server picker (eg synapse-oidc.lab.element.dev)
Or configuring default_server_config.m.authentication in config.json

"default_server_config": {
        "m.homeserver": {
            "base_url": "https://synapse-oidc.lab.element.dev"
        },
        "org.matrix.msc2965.authentication": {
            "issuer": "https://auth-oidc.lab.element.dev/"
        }

    },

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: Validate m.authentication configuration (#3419). Contributed by @kerryarchibald.

[germain-gg]

Globally looks good, added a few comments that I believe need to be addressed before merging this 👍

[germain-gg]

Thank you, looks good to me!