Add support for shared VPCs

[mbtiii]

Last year, MWAA released support for Airflow environments in shared VPCs, as per this blog post: https://aws.amazon.com/blogs/big-data/introducing-shared-vpc-support-on-amazon-mwaa/

The AWS Terraform provider has also been updated to support the endpoint_management attribute in MWAA. This is one of the attributes required to make shared VPCs work with Airflow. For some reason, this attribute isn't documented in the provider docs on the Terraform website, but it is documented in the provider repo itself.

With these things already in place, it should be possible to support using shared VPCs in MWAA using Terraform. This would help out organizations who define their VPCs in a central location and share them with other accounts.