[Platform] Change Platform create on-prem provider form to upload .pem file

[fsayedyb]

Description : Currently, when creating an on-prem provider, the ssh private key (.pem file) is required to be cut and pasted into a field on the form. This is error-prone and often results in universe failing to create, with errors which are difficult to find for new users. Other Platform forms simply require the primary key .pem file to be uploaded directly, thus eliminating the opportunity for error.

Feature request: Please change the on-prem provider create form to work this way (uploading the .pem file directly).
the other provider tabs (screenshot attached for AWS create form) upload the key file, so I think for consistency the best approach is to do this for the on-prem form as well.