Tried the branch, here are some observations:

@FeignClient(name = "something") is turned into @FeignClient(configuration = com.example.demo.FeignConfigExample.class, name = "something")

I would prefer:

• to not have a fully qualified class name appearing in the annotation, but adding an import statement as well
• add the attribute to the end of the already existing list of attributes (in case there are already attributes)

Adding the class to an existing configuration attribute works nicely.

Everything else looks awesome!

There is an OR recipe for adding an attribute to an annotation so i thought it'd be a quick win and would be the starting point as well for turning this into the JDT refactoring... Seems that rewrite sorts attributes in alphabetic order... it sucks that it puts an FQN though... I could have sworn that i saw code in the recipe using simple names rather than FQNs - I'll investigate, hopefully I just misuse the recipe...
I also spotted a bit of a regression when deserializing a recipe without JsonCreator constructor and this PR would fix that...

FQN should be fixed with this: openrewrite/rewrite#6786