Consolidate SSO redirects through `/_matrix/client/v3/login/sso/redirect(/{idpId})` (#17972)

Consolidate SSO redirects through
`/_matrix/client/v3/login/sso/redirect(/{idpId})`

Spawning from
https://github.com/element-hq/sbg/pull/421#discussion_r1859497330 where
we have a proxy that intercepts responses to
`/_matrix/client/v3/login/sso/redirect(/{idpId})` in order to upgrade
them to use OAuth 2.0 Pushed Authorization Requests (PAR). Instead of
needing to intercept multiple endpoints that redirect to the
authorization endpoint, it seems better to just have Synapse consolidate
to a single flow.


### Testing strategy

1. Create a new OAuth application. I'll be using GitHub for example but
there are [many
options](https://github.com/matrix-org/synapse/blob/be65a8ec0195955c15fdb179c9158b187638e39a/docs/openid.md).
Visit https://github.com/settings/developers -> **New OAuth App**
    - Application name: `Synapse local testing`
    - Homepage URL: `http://localhost:8008`
- Authorization callback URL:
`http://localhost:8008/_synapse/client/oidc/callback`
 1. Update your Synapse `homeserver.yaml`
    ```yaml
    server_name: "my.synapse.server"
    public_baseurl: http://localhost:8008/
    listeners:
      - port: 8008
        bind_addresses: [
          #'::1',
          '127.0.0.1'
        ]
        tls: false
        type: http
        x_forwarded: true
        resources:
          - names: [client, federation, metrics]
            compress: false
    
    # SSO login testing
    oidc_providers:
      - idp_id: github
        idp_name: Github
        idp_brand: "github"  # optional: styling hint for clients
        discover: false
        issuer: "https://github.com/"
        client_id: "xxx" # TO BE FILLED
        client_secret: "xxx" # TO BE FILLED
authorization_endpoint: "https://github.com/login/oauth/authorize"
        token_endpoint: "https://github.com/login/oauth/access_token"
        userinfo_endpoint: "https://api.github.com/user"
        scopes: ["read:user"]
        user_mapping_provider:
          config:
            subject_claim: "id"
            localpart_template: "{{ user.login }}"
            display_name_template: "{{ user.name }}"
    ```
1. Start Synapse: `poetry run synapse_homeserver --config-path
homeserver.yaml`
1. Visit
`http://localhost:8008/_synapse/client/pick_idp?redirectUrl=http%3A%2F%2Fexample.com`
 1. Choose GitHub
1. Notice that you're redirected to GitHub to sign in
(`https://github.com/login/oauth/authorize?...`)

Tested locally and works:

1.
`http://localhost:8008/_synapse/client/pick_idp?idp=oidc-github&redirectUrl=http%3A//example.com`
->
1.
`http://localhost:8008/_matrix/client/v3/login/sso/redirect/oidc-github?redirectUrl=http://example.com`
->
1.
`https://github.com/login/oauth/authorize?response_type=code&client_id=xxx&redirect_uri=http%3A%2F%2Flocalhost%3A8008%2F_synapse%2Fclient%2Foidc%2Fcallback&scope=read%3Auser&state=xxx&nonce=xxx`

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