Supporting multiple LLMs in auto-config

Currently, proprietary or open models running in Google Vertex can be accessed through the Vertex Gemini module, respectively using the OpenAI API.

#################################
# Google Vertex AI Gemini
#################################
spring.ai.vertex.ai.gemini.project-id=<project>
spring.ai.vertex.ai.gemini.location=us-central1
spring.ai.vertex.ai.gemini.chat.options.model=gemini-2.0-flash-001
spring.ai.vertex.ai.gemini.transport=grpc

#################################
# OpenAI API VertexAI
#################################
spring.ai.openai.api-key=abc123
spring.ai.openai.vertex.ai.gemini.project-id=<project>
spring.ai.openai.vertex.ai.gemini.location=us-central1
spring.ai.openai.vertex.ai.chat.options.model=meta/llama3-405b-instruct-maas
spring.ai.openai.vertex.ai.chat.base-url=<baseURL>
spring.ai.openai.vertex.ai.chat.completions-path=/chat/completions
spring.ai.openai.vertex.ai.chat.options.max-tokens=1024

The problem - there is no ability to auto-config more than 1 Gemini or open model in Vertex at this time.

The proposal - introduce the ability to add a in the auto-config, to allow for multiple configurations

spring.ai.vertex.ai.gemini.<gemini-flash>.project-id=<project>
spring.ai.vertex.ai.gemini.<gemini-flash>.location=us-central1
spring.ai.vertex.ai.gemini.<gemini-flash>.chat.options.model=gemini-2.0-flash-001
spring.ai.vertex.ai.gemini.<gemini-flash>.transport=grpc

spring.ai.vertex.ai.gemini.<gemini-pro>.project-id=<project>
spring.ai.vertex.ai.gemini.<gemini-pro>.location=us-central1
spring.ai.vertex.ai.gemini.<gemini-pro>.chat.options.model=gemini-1.5-001
spring.ai.vertex.ai.gemini.<gemini-pro>.transport=grpc

Workaround - not using auto-config and manually configuring each model in use

CC @tzolov

Start by tracing the Vertex Gemini module and the OpenAI API VertexAI auto-configuration described in the issue. Determine how the current single-model properties are bound and instantiated, then verify that separate model-family properties can create multiple configurations without breaking existing configuration behavior.