Feature/spring integration support

[dschulten]

What's this PR do/fix?

Adds support for spring-integration-webmvc and spring-integration-webflux

Are there unit tests? If not how should this be manually tested?

There are unit tests, manual tests are possible with the related PR springfox/springfox-demos#57

Any background context you want to provide?

Some changes to the existing code were necessary.
In Spring-Integration, the IntegrationRequestMappingHandlerMapping and WebFluxIntegrationRequestMappingHandlerMapping are initalized later than other HandlerMappings. Therefore I changed DocumentationPluginBootstrapper so that it disables itself in favor of SpringIntegrationDocumentationPluginBootstrapper which fires on ContextRefreshedEvent, and let both derive from a common base class for the actual bootstrapping work. Also, the existing WebMvc and WebFlux RequestHandlerProvider implementations needed to be told to leave the spring-integration handler mappings alone.
The spring-restdocs support is realized by a standard plugin, the SpringRestDocsOperationBuilderPlugin.

What are the relevant issues?

#550
#2754 closed in favor of this PR.
#797

[codecov]

Codecov Report

Merging #2793 into master will decrease coverage by 0.35%.
The diff coverage is 85.08%.

@@             Coverage Diff              @@
##             master    #2793      +/-   ##
============================================
- Coverage     94.78%   94.42%   -0.36%     
- Complexity     3161     3219      +58     
============================================
  Files           354      364      +10     
  Lines          8067     8300     +233     
  Branches        608      616       +8     
============================================
+ Hits           7646     7837     +191     
- Misses          271      305      +34     
- Partials        150      158       +8

Impacted Files	Coverage Δ	Complexity Δ
...ngIntegrationDocumentationPluginsBootstrapper.java	0% <0%> (ø)	0 <0> (?)
...umentation/spring/web/SpringfoxTemplateFormat.java	0% <0%> (ø)	0 <0> (?)
...SpringIntegrationWebMvcRequestHandlerProvider.java	100% <100%> (ø)	5 <5> (?)
...pringIntegrationWebFluxRequestHandlerProvider.java	100% <100%> (ø)	5 <5> (?)
...ng/web/SpringIntegrationWebFluxRequestHandler.java	100% <100%> (ø)	6 <6> (?)
...ing/web/SpringIntegrationWebMvcRequestHandler.java	100% <100%> (ø)	6 <6> (?)
...ringIntegrationNotPresentInClassPathCondition.java	44.44% <44.44%> (ø)	3 <3> (?)
...ing/web/plugins/WebFluxRequestHandlerProvider.java	94.44% <75%> (-5.56%)	6 <1> (+1)
.../plugins/SpringRestDocsOperationBuilderPlugin.java	83.07% <83.07%> (ø)	16 <16> (?)
.../web/plugins/DocumentationPluginsBootstrapper.java	91.3% <90.9%> (-4.85%)	10 <10> (-8)
... and 13 more

[dilipkrish]

Thank you @dschulten, much appreciated 🙇. I did some re-formatting, to match current coding style, apologies for not sharing that publicly. At a high level

• 2 space indents (that was the preference @adrianbk and I agreed on)
• splatting all static imports
• splatting regular imports only if it exceeds 100 classes from that package (a way to make it explicit)