New Resource: `azurerm_eventgrid_namespace`

[catriona-m]

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Description

Adding azurerm_eventgrid_namespace as a new resource.

PR Checklist

• [x] I have followed the guidelines in our Contributing Documentation.
• [x] I have checked to ensure there aren't other open Pull Requests for the same update/change.
• [x] I have checked if my changes close any open issues. If so please include appropriate closing keywords below.
• [x] I have updated/added Documentation as required written in a helpful and kind way to assist users that may be unfamiliar with the resource / data source.
• [x] I have used a meaningful PR title to help maintainers and other users understand this change and help prevent duplicate work. For example: “resource_name_here - description of change e.g. adding property new_property_name_here”

Testing

• [ ] My submission includes Test coverage as described in the Contribution Guide and the tests pass. (if this is not possible for any reason, please include details of why you did or could not add test coverage)

Running tests in TC

Change Log

Below please provide what should go into the changelog (if anything) conforming to the Changelog Format documented here.

• New Resource: azurerm_eventgrid_namespace [GH-27682]

This is a (please select all that apply):

• [ ] Bug Fix
• [x] New Feature (ie adding a service, resource, or data source)
• [ ] Enhancement
• [ ] Breaking Change

Related Issue(s)

Fixes #24079

[!NOTE] If this PR changes meaningfully during the course of review please update the title and description as required.