Add OPAAutoConfiguration to define common beans

[ali-jalaal]

Expected behaviour

Define common beans as part of an auto-configuration to reduce manual configurations on clients side.

Actual behaviour

Clients should define objects/beans in their code to use the library, such as OPAClient and OPAAuthorizationManager.
It would be nice to have a more streamlined experience by defining pre-configured beans in an @AutoConfiguration class
which will be created in client applications by adding that can be used by clients.

How to improve

Define OPAAutoConfiguration class that will create the required beans:

@AutoConfiguration
@EnableConfigurationProperties(OpaProperties.class)
public class OpaAutoConfiguration {

    @Bean
    @ConditionalOnMissingBean(OPAClient.class)
    public OPAClient opaClient(OpaProperties opaProperties) {
        return new OPAClient(opaProperties.getUrl());
    }

    @Bean
    @ConditionalOnMissingBean(OPAAuthorizationManager.class)
    public OPAAuthorizationManager opaAuthorizationManager(OpaProperties opaProperties) {
        return new OPAAuthorizationManager(opaClient(opaProperties), opaProperties.getPath());
    }
}

Then, clients can use the library without defining OPAClient and OPAAuthorizationManager beans/objects:

@Configuration
@EnableWebSecurity
public class SecurityConfig {
    @Bean
    SecurityFilterChain securityFilterChain(HttpSecurity http, OPAAuthorizationManager opaAuthorizationManager) throws Exception {
        http.authorizeHttpRequests(authorize -> authorize.anyRequest().access(opaAuthorizationManager));
        return http.build();
    }
}

Moreover, considering @ConditionalOnMissingBean annotations, clients can define their own OPAClient and OPAAuthorizationManager beans. Fewer clients would be affected by changing the library's internal structure (e.g. constructors). Furthermore, OPAProperties can be used to reference externalized properties (depends on #21).

[chendrix]

This looks good to me. Seems like there's very little risk, and it maintains backwards compatibility.