refactor(OAuth2): groovy to java conversion for OAuth module (#1871)

* refactor(OAuth2): groovy to java conversion for OAuth module

Validated and tested using google and github OAuth.

* Add specific imports and remove *

* Remove DefaultGroovyMethods

* Add StructuredArguments.entries

* Rename safeGet to toStringOrNull. toStringOrNull returns null when the input is null. Add javadoc

* Replace details.getOrDefault with Optional to handle null cases

* Add log statement from groovy code

* Use AssertJ

* First checks log.isDebugEnabled() to avoid unnecessary String concatenation

* Add non empty string check

* Add test for SpinnakerUserInfoTokenServices#isServiceAccount

* Refactor SpinnakerUserInfoTokenServices to use constructor injection

- Replaced member injection with constructor injection
- Improved encapsulation by removing direct member access
- Updated tests to pass dependencies via constructor

* Preserve method call verifications in tests

* Refactor to use constructor injection

- Removed  annotations from individual members
- Added  to the all-args constructor
- Removed no-arg constructor to enforce dependency injection
- Improved clarity and testability of the class

* Add specific imports and remove *

* Move SpinnakerProviderTokenServices to constructor argument and make it optional using Optional<SpinnakerProviderTokenServices>

Author: rahul-chekuri

gate-oauth2/gate-oauth2.gradle

@@ -2,6 +2,7 @@ dependencies {
   implementation project(":gate-core")
   implementation "com.netflix.spectator:spectator-api"
   implementation "io.spinnaker.fiat:fiat-api:$fiatVersion"
+  implementation "io.spinnaker.fiat:fiat-core:$fiatVersion"
   implementation "io.spinnaker.kork:kork-exceptions"
   implementation "io.spinnaker.kork:kork-retrofit"
   implementation "io.spinnaker.kork:kork-security"