Support extending username password authentication

[JohnNiang]

What type of PR is this?

/kind feature
/area core
/area plugin

What this PR does / why we need it:

Plugin developers are able to define own UsernamePasswordAuthenticationManager to take charge of username password authentication.

1. If the manager fails to handle, the default authentication manager will be used.
2. If the manager returns Mono.empty(), the default authentication manager will be used.

For example:

@Component
public class LdapAuthenticationManager
    extends UserDetailsRepositoryReactiveAuthenticationManager
    implements UsernamePasswordAuthenticationManager {

    public LdapAuthenticationManager(ReactiveUserDetailsService userDetailsService) {
        super(userDetailsService);
    }

    @Override
    protected Mono<UserDetails> retrieveUser(String username) {
        return super.retrieveUser(username);
    }
}

Which issue(s) this PR fixes:

See #4207 (comment) for more.

Does this PR introduce a user-facing change?

提供用户名密码认证扩展

[codecov]

Codecov Report

Merging #4265 (6986546) into main (e98aec3) will increase coverage by 0.11%.
The diff coverage is 50.00%.

@@             Coverage Diff              @@
##               main    #4265      +/-   ##
============================================
+ Coverage     60.16%   60.28%   +0.11%     
- Complexity     2418     2426       +8     
============================================
  Files           366      367       +1     
  Lines         12617    12637      +20     
  Branches        906      906              
============================================
+ Hits           7591     7618      +27     
+ Misses         4585     4577       -8     
- Partials        441      442       +1     

Impacted Files	Coverage Δ
...ernamePasswordDelegatingAuthenticationManager.java	31.25% <31.25%> (ø)
...ntication/login/UsernamePasswordAuthenticator.java	44.94% <100.00%> (+2.59%)	⬆️

... and 5 files with indirect coverage changes

[JohnNiang]

/cc @liuchangfitcloud

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[guqing]

需要合并一下冲突

[liuchangfitcloud]

@JohnNiang extensionpoint-definitions.yaml type:SINGLE_INSTANCE Not Found

[JohnNiang]

感谢提醒！已经在 eb28a02 修复。

[guqing]

/approve

[ruibaby]

/lgtm

[f2c-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: guqing, ruibaby

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