Change encrypted provider to require an OS keyring

[dmjb]

The encrypted provider attempts to insert its decryption password into the os keyring using the zalando/go-keyring library. If no keyring is available, the CLI will prompt the user to enter the decryption password each time the tool is used. This was a helpful behaviour for the CLI, but we cannot prompt a user to enter a password for the API/UI workflow.

There are a number of potential ways to approach this, but the most simple one is to make the OS keyring a hard requirement for the encrypted provider. This should be enforced at the secret provider setup time (see previous ticket) - check to see if a keyring is available, and if not, then we should error out and display a message telling the user that the keyring is not available.