Java: Add Agent/Assistant support for OpenAI

[johnoliver]

Investigate how to integrate assistants into the Java library

### Tasks
- [ ] https://github.com/microsoft/semantic-kernel-java/issues/181
- [ ] https://github.com/microsoft/semantic-kernel-java/issues/182
- [ ] https://github.com/microsoft/semantic-kernel-java/issues/183
- [ ] https://github.com/microsoft/semantic-kernel-java/issues/184
- [ ] https://github.com/microsoft/semantic-kernel-java/issues/185
- [ ] https://github.com/microsoft/semantic-kernel-java/issues/207