aws_rds_global_cluster: Add `endpoint` attribute for DB writer endpoint

[ppapishe]

Description

AWS Started supporting global endpoint for global aurora clusters. This PR is to basically add the new endpoint attribute as a new output attribute.

This basically solves https://github.com/hashicorp/terraform-provider-aws/issues/39862

Relations

Closes #39862

References

Output from Acceptance Testing

% make testacc TESTS=TestAccXXX PKG=ec2

...

[github-actions[bot]]

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[github-actions[bot]]

Thank you for your contribution! :rocket:

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[YakDriver]

@ppapishe Thanks for your contribution! I cleaned up a few things and will work on getting this merged.