feat: add OAuth2 sample applications and configurations

[myProjectsRavi]

Description

Resolves #1195 by adding comprehensive OAuth2 sample implementations for Spring Boot Admin.

Changes Made

New Modules

• spring-boot-admin-sample-oauth2 - Admin Server with OAuth2 login integration
• spring-boot-admin-sample-oauth2-client - Monitored application configured as OAuth2 Resource Server

Key Components

Admin Server (OAuth2 Client):

• OAuth2SecurityConfig - OAuth2 login configuration with oauth2Login()
• OAuth2RestTemplateConfig - RestTemplate with OAuth2 client credentials support
• SpringBootAdminOAuth2Application - Main application with @EnableAdminServer

Client App (OAuth2 Resource Server):

• OAuth2ResourceServerConfig - JWT-based resource server protecting actuator endpoints
• OAuth2ClientApplication - Sample monitored application with BufferingApplicationStartup

Configuration Examples

• Complete OAuth2 client registration setup (client-credentials flow)
• Resource server JWT validation configuration
• Environment variable support via application-oauth2.yml
• Actuator endpoints exposure for monitoring

Module Integration

• Updated parent samples POM to include both new modules
• Follows existing project structure and naming conventions
• Compatible with current Spring Boot 3.x and Spring Security patterns

Testing

• ✅ Clean build passes (mvn clean install)
• ✅ Both applications start successfully
• ✅ OAuth2 client can obtain tokens using client-credentials flow
• ✅ Resource server validates JWT tokens for actuator endpoints
• ✅ Admin server can monitor OAuth2-protected client applications

Documentation

Includes example configurations for:

• Auth0 OAuth2 provider setup
• Client credentials flow
• JWT resource server validation
• Environment-based configuration

Closes #1195