Add custom tags to the Datadog integration

What does this MR do and why?

Add a new input field Tags to the settings for Datadog integration. This is to provide custom tags to Datadog for spans from the pipelines at which the setting is configured.

We send this to our intake through a query parameter.

Screenshots or screen recordings

How to set up and validate locally

Just checkout the branch and test the datadog integration with it. The tags typed should appear in the spans in Datadog.

If you don't have a datadog testing environment, you can:

1. Checkout the branch
2. Create a project with a datadog integration including tags
3. Open a console railse console
4. Find the webhook URL that the integration has created: Project.find(PROJECT_ID).datadog_integration.service_hook.url
5. Observe that the URL includes a tags parameter

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

I have updated the docs and added a changelog entry but it's not hidden with a feature flag. I think it's small enough to release directly if you agree. That's what we have done in our side at least.