RFR: 1855: Make it configurable which repositories merges should be allowed from [v2]

[Zhao Song]

> In this patch, PR bot has been added with the capability to control which repositories can be the source for Merge-style pull requests.
> 
> By default, all repositories are allowed.
> 
> However, if it is desired to configure "allowed" repos, the configuration can be added to the PR bot configuration as follows:
> 
> {
>   "pr": {
>       "repositories": {
>           "repo1": {
>                "mergeSources": [openjdk/repo2, openjdk/repo3]
>           }
>        }
>   }
> }
> 
> 
> And the repo itself can always be the source of Merge-style pull requests.

Zhao Song has updated the pull request incrementally with two additional commits since the last revision:

 - fix test
 - Update bots/pr/src/main/java/org/openjdk/skara/bots/pr/CheckWorkItem.java
   
   Co-authored-by: Erik Joelsson <37597443+erikj79 at users.noreply.github.com>

-------------

Changes:
  - all: https://git.openjdk.org/skara/pull/1503/files
  - new: https://git.openjdk.org/skara/pull/1503/files/05b86921..e4e766b6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1503&range=01
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1503&range=00-01

  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/skara/pull/1503.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1503/head:pull/1503

PR: https://git.openjdk.org/skara/pull/1503