Warn about PRs that leave TODOs related to the Jira issue being fixed

[yrodiere]

See for example hibernate/hibernate-search#2173 : it clearly attempts to fix HSEARCH-3729 (see commit messages), but left a // TODO HSEARCH-3729 in the code:

https://github.com/hibernate/hibernate-search/blob/b422fef1ca93aadf2359ce918df337f4c459ca5e/mapper/pojo-base/src/main/java/org/hibernate/search/mapper/pojo/massindexing/impl/PojoMassIndexingBatchCoordinator.java#L270-L272

Ideally, when Jira integration is enabled with the bot, and a PR is submitted, the bot would:

1. gather the keys of all Jira issues fixed in the PR (it already does)
2. search through the code (at the PR head) for FIXME <the issue key / TODO <the issue key
3. if anything was found, leave a review with annotations on the relevant sections of the code, or fail a status check, or anything else that warns contributors that something seems wrong.

Note that searching simply for the issue key in the code would lead to false positives (e.g. if a test references the issue), so it's not a good idea.