Enhance experience when multiple authentication methods are available

[matrixbot]

This issue was originally created by @pmaier1 at matrix-org/matrix-authentication-service#1874.

Currently, the experience is good enough when:

• there is only one IDP and no password auth, because the upstream IDP login happens right away
• there is only password auth, because the user is presented with a username/login form

We need to enhance the experience when there are multiple upstream IdPs, and/or a mix of upstream IdPs and local password authentication options.

Tasks

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1. Administrators can give a human-readable name to their IdPs #2065
   A-Upstream-OAuth T-Enhancement +2
2. Users can link/unlink multiple IdPs to their account #3244
   0 of 4
   A-Account-Management A-Frontend A-GraphQL A-Upstream-OAuth +4
3. Users can discover which IdP they used from their username (?)

[sandhose]

Relevant Figma designs for this: https://www.figma.com/design/66PalsLxjI7v0UAWF1bGE0/Compound-Matrix-Auth-Service-(MAS)-Templates?node-id=569-44518&t=8iPMIX7HjagMTqUq-4