Feature request: Control user mappings for codeowners

[YassineElbouchaibi]

Context:

User Mappings allow mapping between your "source control teams/users" and "Sentry Teams/users", using external team/user mappings. Sentry uses the "external team/user mappings" to convert the owners in your CODEOWNERS file into their equivalents in Sentry.

This is very similar to Code Mappings which is already implemented here: https://github.com/jianyuan/terraform-provider-sentry/blob/main/sentry/resource_sentry_organization_code_mappings.go

The following API endpoints could be used:

• https://docs.sentry.io/api/integrations/create-an-external-user/
• https://docs.sentry.io/api/integrations/delete-an-external-user/
• https://docs.sentry.io/api/integrations/update-an-external-user/