Implement both StaticTokenCredential and OnBehalfOfCredential for java

[v-hongli1]

1. Implement both StaticTokenCredential and OnBehalfOfCredential for java
2. Added and changed files:
   1. java/pom.xml
   2. java/src/main/java/com/azure/identity/extensions/CredentialBuilderBase.java
   3. java/src/main/java/com/azure/identity/extensions/OnBehalfOfFlowCredential.java
   4. java/src/main/java/com/azure/identity/extensions/OnBehalfOfFlowCredentialBuilder.java
   5. java/src/main/java/com/azure/identity/extensions/StaticTokenCredential.java
   6. java/src/main/java/com/azure/identity/extensions/StaticTokenCredentialBuilder.java
   7. java/src/main/java/com/azure/identity/extensions/implementation/HttpPipelineAdapter.java
   8. java/src/main/java/com/azure/identity/extensions/implementation/IdentityClient.java
   9. java/src/main/java/com/azure/identity/extensions/implementation/IdentityClientBuilder.java
   10. java/src/main/java/com/azure/identity/extensions/implementation/util/ValidationUtil.java
   11. java/src/test/java/com/azure/identity/extensions/OnBehalfOfFlowCredentialTests.java
   12. java/src/test/java/com/azure/identity/extensions/StaticTokenCredentialTests.java
   13. java/src/test/java/com/azure/identity/extensions/implementation/IdentityClientTests.java
   14. java/src/test/java/com/azure/identity/extensions/util/TestUtils.java

@jongio for notification.

[jongio]

@joshfree

[joshfree]

Thanks @hongli750210 and @jongio; I've saved a link to this PR in the Azure.Identity OneNote for later in the semester