RFR: 2480: Add new PR command /ping [v3]

[Zhao Song]

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

Zhao Song has updated the pull request incrementally with one additional commit since the last revision:

  review comment

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Changes:
  - all: https://git.openjdk.org/skara/pull/1716/files
  - new: https://git.openjdk.org/skara/pull/1716/files/2239c18f..206d55e6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1716&range=02
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1716&range=01-02

  Stats: 10 lines in 4 files changed: 2 ins; 0 del; 8 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