Provide users with Login Selector screen when multiple authentication providers are enabled

[kobelb]

We have limited support for multiple auth providers in Kibana. We're able to use the basic or token auth providers with either: SAML, Kerberos or Open ID Connect. But, we aren't currently able to use SAML with Kerberos or Open ID Connect. Per @jkakavas's original suggestion in #18366 if we provided a splash screen where the user could choose which auth provider they wish to use, it would greatly improve this situation.

[elasticmachine]

Pinging @elastic/kibana-security

[azasypkin]

++, adding this will become less painful once we finish NP migration PR. There I'm going to make login attempt generic so that if we want some specific provider to perform an initial login, no matter what authentication providers we have enabled and in which order ([saml, basic, kerberos, oidc]), we can say that this particular login request should be processed only by provider X.

[azasypkin]

Here are the 4 states of the Login page I can think of (very rough prototypes, just to illustrate the idea):

1. The page that's displayed when login selector is disabled (xpack.security.authc.selector.enabled: false) and only Basic/Token authentication provider is enabled (current login page):
   

2. The page that's displayed when login selector is enabled (xpack.security.authc.selector.enabled: true), but neither Basic nor Token authentication provider is enabled (in the example only two SAML providers are enabled):
   

3. The page that's displayed when login selector is enabled (xpack.security.authc.selector.enabled: true), and either Basic or Token authentication provider is enabled as well:
   

4. The page that's displayed when login selector is enabled (xpack.security.authc.selector.enabled: true), but none of the providers is enabled:

[ryankeairns]

@azasypkin Here is the mockup we reviewed over zoom. Notable changes are:

• Use EuiButtons instead of a list; add the fullWidth prop
• Swap out the icons to the user iconType; use logoCloud for Elastic Cloud SAML, if possible
• Keep the ----- OR ----- as you had it in your original mockup

Thank you, let me know if any other questions arise.