Add customizer interface for spring boot integration

[koenpunt]

This to allow customization of the client, without having to manually create the client.

This also includes the changes from #433.

[koenpunt]

This implementation was inspired by Spring Boot's autoconfiguration for the webclient;

https://github.com/spring-projects/spring-boot/blob/d55cd3b46fc33e9c2806fac0079993954c62a7d3/spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/reactive/function/client/WebClientAutoConfiguration.java#L54-L56

[edsonjab]

Hi @koenpunt all your changes looks pretty well, could you help us to run mvn spotless:apply to fix the tests, please?

[koenpunt]

@edsonjab done!

[edsonjab]

All works.

[MichaelGHSeg]

Our development and maintenance efforts have shifted to the new Analytics-Kotlin library. We suggest migrating when you can and we are more likely to accept interface suggestions over there. You're also welcome to continue using your fork of this repo of course :)

[koenpunt]

We've used the analytics kotlin library before, but it's missing features and we actually switched to the Java library. I think as long as the kotlin library is mobile device oriented and keeps state during the lifetime of an instance the java project shouldn't be abandoned.

[koenpunt]

Also, Java-only projects are still a thing, and I can't imagine them adding kotlin just for the Segment integration. @MichaelGHSeg it would be greatly appreciated if this could be reconsidered.

[didiergarcia]

We've used the analytics kotlin library before, but it's missing features and we actually switched to the Java library. I think as long as the kotlin library is mobile device oriented and keeps state during the lifetime of an instance the java project shouldn't be abandoned.

Hi @koenpunt, please let us know what features are missing in the analytics-kotlin version. We're super interested in making switching easier.

As far as adding analytics-kotlin I think it's only kotlin-stdlib, kotlin-coroutines, and analytics-kotlin that would be needed.

We recently added a analytics.shutdown() for clean termination in containerized environments and will be working on an enhancement to allow the library to work in a stateless manner later this year.

[koenpunt]

I think there is too much missing for the Java library to be put in the background. We switched to the Java library, so can't really give any feedback on the kotlin library.
But the main problem with the kotlin library is that it maintains state, which in my opinion only makes sense in an (mobile) app environment, and not a server environment.
And in regards to the kotlin library for Java only projects; it doesn't make sense to require people to add kotlin libraries just for segment, especially when there's fully functional (if not better) Java implementation.

I do think both should be able to exists, and one shouldn't be replacing the other.