JiraConnect: Send feature-flag information

What does this MR do?

Relates to #229347 (closed)

This adds a feature-flag module to our JiraConnect integration, with support for sending updates.

The feature is guarded by a feature flag: :jira_sync_feature_flags

QA instructions

• Enable the :sync_jira_deployments feature flag in your rails console.
• Install or update your JiraConnect installation (see https://gitlab.com/gitlab-org/gitlab/blob/master/doc/development/integrations/jira_connect.md)
• Link the installation to a namespace you want to test in.
• Create one or more Jira issues, taking note of their issue keys (e.g. FOO-123).
• Create one or more feature flags that mention these issue keys in their description.
• Verify that the feature flag appears in Jira (see screenshots)

Console based QA:

the request responses can be inspected in the logs, or at the terminal with:

ff = Operation::FeatureFlag.last
p = ff.project
s = JiraConnect::SyncService.new(p)
s.execute(feature_flags: [ff])

Screenshots (strongly suggested)

Table view:

Issue view:

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry - not needed, since this is behind a feature flag.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.