[Do Not Merge Yet] Spring Boot 4 compatibility

[paulbakker]

This branch builds against Spring Boot 4 / Spring Framework 7.

The main changes are:

1. AutoConfiguration has be restructured in Spring Boot. This primarily affects tests in Spring AI.
2. Spring-retry is now part of Spring Framework, but with an updated API
3. HttpHeaders no longer implements MultiMap. I updated Spring AI to use HttpHeaders directly.
4. Elastic search now uses the Rest5Client

There are a few TODOs in this PR, mostly related to some open issues in Spring Boot with the restructuring of RestClient/WebClient. I'll update the PR once the next milestone or RC comes out.

This should not be merged yet since Spring Boot 4 isn't released yet, but it would be very useful to have a seperate release targetting Boot 4 / Spring Framework 7.

[ilayaperumalg]

@paulbakker Thanks very much for the PR!

[paulbakker]

Note that I have disabled the models/spring-ai-huggingface module for now since it relies on Swagger codegen that I think requires work on the codegen side.

[paulbakker]

@ilayaperumalg Would it be possible to get a milestone or snapshot release based on this branch for folks preparing for the Spring Boot 4 release? It's currently one of the blockers.

The dual release (and rebasing of this branch) will likely have to continue for the next several months, so it's worth thinking this through a bit more.

I was also discussing this on Slack with Mark: https://spring-ai-community.slack.com/archives/C08FB2N8M53/p1761155506523019?thread_ts=1760565021.498989&cid=C08FB2N8M53

[ilayaperumalg]

@paulbakker Thanks for following up on this. I agree that we can create a new branch along side main to have the changes required for Spring Boot 4 compatibility which can then become main when we switch to Spring AI 2.0. Until then, we can start publishing snapshot from the interim branch. I hope this works out ok for you.

I am marking this for "discussion" with the team and I'll update here once we make the decision on this(sooner).