Improve FilePath API and Javadoc around validation

[daniel-beck]

One of the causes of https://www.jenkins.io/security/advisory/2021-03-18/#SECURITY-2041 seems to have been the multiple different methods of FilePath to validate certain file masks. One group is suitable for form validation around workspace contents, as it performs a permission check and limits the number of operations, while the other is more suitable for running during a build, and does neither. This PR attempts to clarify this difference.

Additionally, this adds an overload of FilePath#validateAntFileMask that doesn't take an explicit parameter for the file mask bound (which is usually simply the static field with the default value).

Proposed changelog entries

• Developer: Add FilePath#validateAntFileMask(String, boolean) overload for convenience.

Proposed upgrade guidelines

N/A

Submitter checklist

• (If applicable) Jira issue is well described
• Changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developer, depending on the change). Examples
  • Fill-in the Proposed changelog entries section only if there are breaking changes or other changes which may require extra steps from users during the upgrade
• Appropriate autotests or explanation to why this change has no tests
• For dependency updates: links to external changelogs and, if possible, full diffs

Desired reviewers

@mention

Maintainer checklist

Before the changes are marked as ready-for-merge:

• There are at least 2 approvals for the pull request and no outstanding requests for change
• Conversations in the pull request are over OR it is explicit that a reviewer does not block the change
• Changelog entries in the PR title and/or Proposed changelog entries are correct
• Proper changelog labels are set so that the changelog can be generated automatically
• If the change needs additional upgrade steps from users, upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the PR title. (example)
• If it would make sense to backport the change to LTS, a Jira issue must exist, be a Bug or Improvement, and be labeled as lts-candidate to be considered (see query).

[uhafner]

Thanks for the detailed description!

[basil]

This PR is now ready for merge. We will merge it after approximately 24 hours if there is no negative feedback. Please see the merge process documentation for more information about the merge process. Thanks!