RFR: 1870: Make pr processing optional

[Zhao Song]

In this patch, PR bot has been added with the capability to disable PR processing and Commit processing.

By default, PR processing and Commit processing are both enabled.

However, if it is desired to disable PR processing or Commit processing in a repo, the configuration can be added to the PR bot configuration as follows:

{
  "pr": {
      "repositories": {
          "repo1": {
               "processPR": false,
               "processCommit": false
          }
       }
  }
}

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Commit messages:
 - SKARA-1870

Changes: https://git.openjdk.org/skara/pull/1498/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1498&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1870
  Stats: 156 lines in 5 files changed: 145 ins; 1 del; 10 mod
  Patch: https://git.openjdk.org/skara/pull/1498.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1498/head:pull/1498

PR: https://git.openjdk.org/skara/pull/1498