Login Info Endpoint: optimize OAuth IdPs for JSON Response

[adrianhoelzl-sap]

(alternative for #3251)

This PR optimizes the login info endpoint so that it does not fetch all OAuth IdPs when a JSON response is requested.

Prior to the optimization, all OAuth IdPs are queried and then used for determining a potential redirect to the corresponding external IdP. However, for JSON responses the redirect is ignored anyway.

The optimization introduces the following changes to the JSON response, which are in our opinion acceptable:

• showLoginLinks is no longer returned if there is also no SAML IdP with showSamlLink enabled
  • this property is only relevant for the login page and therefore obsolete for the JSON response
  • in the JSON response, only SAML IdPs are part of the idpDefinitions anyway
• the passcode prompt is no longer removed if there is no SAML/OIDC IdP with showSamlLink/showLinkText in the IdP definitions list
  • password grants with a one-time passcode also work with the UAA user store

The first few commits in this PR are refactorings of the endpoint handler, they don't change any behavior. We advise to go through the PR from commit to commit.