Make test run identifying fields required

The new fields introduced in https://github.com/cucumber/messages/pull/102 are optional. This was a tactical move at the time because making them required immediately would prohibit compatibility with any existing messages. Once this change has had plenty of time to be adopted by major Cucumber implementations and reporting tools, we should mark the fields as required.

The fields are:

- `TestRunStarted.id`
- `TestCase.testRunStartedId`
- `TestRunFinished.testRunStartedId`

No file or test is named in the issue. Start by locating the schema definitions for TestRunStarted.id, TestCase.testRunStartedId, and TestRunFinished.testRunStartedId, then review compatibility requirements across the message implementations and reporting tools. Done means all three fields are required without breaking existing consumers.

Written by the indexing model from the issue text.