Sql server/azure connection string parameter for readonly access?
No, there is unfortunately no such connection string parameter.  The "ApplicationIntent=ReadOnly" option is just used by the SQL driver to attempt to route your connection to a read only replica.

I'd suggest that, if there are users or processes that should only have read access to this database, you accomplish this using the built-in SQL Server permissions.  In other words, only add those users / processes to the db_datareader database role if they are accessing tables and view directly.  

If they are accessing data through stored procedures, be careful to only grant them EXECUTE permissions on the read procedures.