RFR: 2480: Add new PR command /ping

[Zhao Song]

This PR is trying to add a new pull request "ping".

The reported wants to have a pull request command that can refresh the timeouts of the pr so that the pr wouldn't be closed by pullRequestPrunerBot.
 
The reported proposed to name the command "keep-alive" with alias "/ping", however, we store command names in an Enum class, so "keep-alive" is invalid. I think we just use "ping" as the command name.

Besides, I would like to let the command trigger a force update of the pr. Currently, there are some cases that skara bot won't re-evaluate the pr automatically and users need to do something to poke it(like editing the pr title), so I think it's better to have a pull request command to trigger the update.

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Commit messages:
 - update
 - SKARA-2480

Changes: https://git.openjdk.org/skara/pull/1716/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1716&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2480
  Stats: 101 lines in 6 files changed: 95 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/skara/pull/1716.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1716/head:pull/1716

PR: https://git.openjdk.org/skara/pull/1716