New ModelRouter module

[hkuhn42]

Issue

Closes langchain4j/langchain4j#4490

Change

Adds a new ModelRouter model with a pluggable ModelRoutingStrategy

• New class ModelRouter
• New interface ModelRoutingStrategy
• ModelRoutingStrategy impl for failover routing
• ModelRoutingStrategy impl for least token usage routing

General checklist

• There are no breaking changes
• I have added unit and integration tests for my change
• I have manually run all the unit tests in all modules, and they are all green
• I have manually run all integration tests in the module I have added/changed, and they are all green

• I have added/updated the documentation
• I have added an example in the examples repo (only for "big" features)
• I have added/updated Spring Boot starter(s) (if applicable)

Checklist for adding new maven module

• I have added my new module in the root pom.xml and langchain4j-community-bom/pom.xml

Checklist for adding new embedding store integration

• I have added a {NameOfIntegration}EmbeddingStoreIT that extends from either EmbeddingStoreIT or EmbeddingStoreWithFilteringIT
• I have added a {NameOfIntegration}EmbeddingStoreRemovalIT that extends from EmbeddingStoreWithRemovalIT

Checklist for changing existing embedding store integration

• I have manually verified that the {NameOfIntegration}EmbeddingStore works correctly with the data persisted using the latest released version of LangChain4j

[Martin7-1]

@hkuhn42 Thanks for the great idea! Could you please run make format to make spotless happy?

BTW, please add some ITs to use real LLM call :)