Draft: Decouple modules from (ok)http client

[langchain4j]

Issue

#1044

Change

This is the first step towards decoupling modules from (ok)http client.
This PR introduces an abstraction layer (in langchain4j-core module) over various http clients.
Main components:

• HttpClient
• HttpClientBuilder
• HttpRequest
• HttpResponse

This PR also provides 2 implementations of HttpClient:

• OkHttpHttpClient (uses okhttp that is currently used in most modules, but does not use retrofit)
• SpringRestClientHttpClient (uses RestClient from Spring)

More implementations (e.g. JDK 11 HttpClient) can be added later.

The first module to migrate is langchain4j-anthropic, this PR decouples it from okhttp.
Users are able to provide a HttpClientBuilder when creating Anthropic*Model. If not provided, default (okhttp) will be loaded via SPI.

With this change, request/response logging is also unified and moved to HttpRequestLogger and HttpResponseLogger. So all modules can now get logging for free.

General checklist

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

• I have added/updated the documentation
• I have added an example in the examples repo (only for "big" features)

Checklist for adding new model integration

• I have added my new module in the BOM

[langchain4j]

Hi @geoand and @ThomasVitale
Could you please take a quick look at this draft? 🙏

[geoand]

Nice!

Right off the bat I would say that I don't believe there should be a Spring specific implementation in the core repo. Spring REST Template is not really something many (any?) non Spring application use.
If anything, there could be a JDK HttpClient module (which would require Java 11+ though).

[geoand]

Also for the Quarkus integration, I don't really think this will make much of a difference as we really want to continue using the declarative clients

[langchain4j]

@geoand thanks for the feedback! Spring specific things will be moved to langchain4j-spring repo (once this PR is about to be finalized), there are a few TODOs in the PR.
Regarding JDK 11 HttpClient, I guess it makes total sense, but probably can be delayed a bit, since there are so many other higher-prio issues.