Developer: add generative-ai-app-builder client REST api developer config - ideally in spring boot

https://github.com/obrienlabs/magellan/issues/26

see HttpClient in https://github.com/GoogleCloudPlatform/pubsec-declarative-toolkit/issues/645

https://github.com/ObrienlabsDev/rest-client-java/tree/main

Client request: generative-ai-app-bulder (Vertex AI search and conversation) REST API usage from containerized spring boot app
Review use of ADC with the client https://cloud.google.com/docs/authentication/provide-credentials-adc
ADC is used by https://github.com/cloud-quickstart/gcp-landing-zone/blob/main/gcp-landing-zone-deploy/src/main/java/zone/gcp/landing/Cli.java#L74

verify firewall as well - 443/8443 for https and https://www.gstatic.com/ipranges/goog.json for IPs

```
GoogleCredentials credential =
GoogleCredentials.getApplicationDefault()
.createScoped(Collections.singleton(IamScopes.CLOUD_PLATFORM));
```

Require both local dev (STS eclipse) and containerized auth/configs/API(open API/swagger) setup

https://cloud.google.com/generative-ai-app-builder/docs/locations#specify_a_multi-region_for_your_data_store

see #559