How to install tds_fdw on AWS RDS for SQL Server
If RDS doesn't allow it, then you can't do it on RDS.  The straightforward solution is not to use RDS if it doesn't support the features you need.
But RDS does allow postgres_fdw.  So the convoluted solution is to run your own postgres server (perhaps running it on an EC2 instance) which does supports tds_fdw, and then connect to that from RDS and chain the foreign tables.