Support multiple SSO identity providers during login/UIA flow

[richvdh]

We would like to give synapse support for multiple OIDC identity providers. As a first step:

• the user would be prompted to choose one from the /_matrix/client/r0/login/sso/redirect page
  • give synapse the concept of an "identity provider", and refactor existing CAS, SAML and OIDC impls to match it
  • redirect to an IdP picker when more than one IdP exists
  • serve an IdP picker based on a template
  • support multiple OIDC providers, each as a separate IdP (Land support for multiple OIDC providers #9110)
  • write some tests
• for UIA, the user would be taken to whichever OIDC provider they had registered with. (UI Auth via SSO: redirect the user to an appropriate SSO. #9081)
• login fallback needs to support picking a provider too actually, this is better done once synapse supports MSC2858; moved to Support picking an SSO provider during login fallback #9013

[richvdh]

(I've rescoped this to be just the initial support, taking proper MSC2858 support to #8928)

[richvdh]

#8876 is a prerequisite here.

[richvdh]

first parts of this are in #9017