TL;DR: support method=POST for the /blue/rest/classes API so very long values for "classList" can be sent.

The new capabilities code on the frontend (JENKINS-37519) may potentially call the /blue/rest/classes?q=classList API with a very long "classList" parameter (in some cases over 1700 chars long). Some browsers have problems with URL's over 2000 characters, although it's unclear whether that applies to fetch / XHR calls or just URLs pasted into the address bar UI. To be safe it would be good to support the POST method so we can send large "classList" values in the body and avoid any potential problems.

Also fix these issues:

• For pipeline-related things, I think it would be good to refactor the string values into static string constants in a single class (or possibly an enum( to ensure consistent values are used throughout and make it easier to see what capabilities are available.
• Fix typo in BranchImpl.java ("io.jenkins.blueocean.rest.impl.pipeline.PullReuqest")