Previously, REST Docs' JUnit Jupiter extension,
RestDocumentationExtension, stored its own state. Since the code
was written I have learned that this isn't recommended and that the
extension context's store should be used instead. Up until now, we'd
got away with the extension being stateful, but the introduction of
parallel test execution has highlighted the problem.
This commit updates RestDocumentationExtension to use the store from
the execution context rather than storing its own state. An unwanted,
but necessary, side-effect of this is that RestDocumentationExtension
itself no longer implements RestDocumentationContextProvider and,
instead, returns a new type rather than itself when asked to resolve
the provider. This is technically a breaking API change, but users
will be unaffected if they have followed the recommended and
documented approach of injecting RestDocumentationContextProvider.
Closes gh-520
99ee889708