Add predicate queued to taskRun to indicate that task is planned to be started in future

[BaLiKfromUA]

Changes

Closes #305

This PR introduces a new taskRun predicate: queued.

Consistent with other queued events, this predicate indicates that a task is waiting for the applicable preconditions to be satisfied before it can start running.

This is needed for two reasons:

1. There is currently a small inconsistency between the predicates available for taskRun and the kinds of work that a task can represent. The testSuiteRun subject from Testing Events already supports a queued predicate. Since a task can represent a test run, adding queued to taskRun seems consistent with the specification:

   ...a set of Tasks to be performed, like performing a build, running some tests, publishing an artifact.

2. There is currently no way to express a taskRun that has been scheduled for future execution but has not started yet. For example, a pipelineRun may already be started and in progress while one of its taskRuns is still queued.

   Note: As of July 2026, there is an ongoing discussion about introducing a separate Scheduler abstraction in CDEvents. Such an addition would not conflict with this PR. In the meantime, the queued predicate can serve as a practical compromise.

As an example for making this contribution, I've been using #145

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Has the primer doc been updated if a design decision is involved
• Have the JSON schemas been updated if the specification changed
• Has spec version and event versions been updated according to the versioning policy
• Meets the CDEvents contributor standards

[BaLiKfromUA]

Validated JSONs locally:

>  cd tools && npm ci && npm run validate
....
taskrun queued: valid
....
50 out of 50 examples are valid