Fix authn delegation behavior

[leleuj]

If we force the authn delegation process on an existing SSO session, the current behavior is unappropriate: the credentials should not be put in the webflow for an existing session as they trigger a login process, erasing the previous authenticated user.

I also changed the visibility of two methods from private to protected for customisation purposes.

[claassistantio]

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[codecov]

Codecov Report

Merging #4643 into 6.1.x will increase coverage by 13.09%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##              6.1.x    #4643       +/-   ##
=============================================
+ Coverage     32.75%   45.84%   +13.09%     
- Complexity     5210     7662     +2452     
=============================================
  Files          2579     2579               
  Lines         52522    52506       -16     
  Branches       4176     4174        -2     
=============================================
+ Hits          17204    24073     +6869     
+ Misses        33994    26562     -7432     
- Partials       1324     1871      +547

Impacted Files	Coverage Δ	Complexity Δ
.../web/flow/DelegatedClientAuthenticationAction.java	53.84% <ø> (+53.84%)	21 <0> (+21)	⬆️
...org/apereo/cas/couchdb/tickets/TicketDocument.java	0% <0%> (-100%)	0% <0%> (-1%)
...g/apereo/cas/services/DynamoDbServiceRegistry.java	0% <0%> (-100%)	0% <0%> (-7%)
...DbMultifactorAuthenticationTrustConfiguration.java	0% <0%> (-100%)	0% <0%> (-4%)
...o/cas/monitor/config/LdapMonitorConfiguration.java	0% <0%> (-100%)	0% <0%> (-3%)
...icket/refreshtoken/OAuth20DefaultRefreshToken.java	0% <0%> (-100%)	0% <0%> (-3%)
...org/apereo/cas/audit/CouchDbAuditTrailManager.java	0% <0%> (-100%)	0% <0%> (-4%)
...s/config/MemcachedTicketRegistryConfiguration.java	0% <0%> (-100%)	0% <0%> (-5%)
...nfig/SurrogateLdapAuthenticationConfiguration.java	0% <0%> (-100%)	0% <0%> (-3%)
...apereo/cas/config/CouchDbYubiKeyConfiguration.java	0% <0%> (-100%)	0% <0%> (-6%)
... and 1232 more

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