fix(sso): change "Decryption private key" behavior to make SAML "Want assertion signed" easier to use

Description

With SAML, when we try to use a private key to sign in (want_assertions_signed enabled), the field "Decryption Private Key" is not used.

Instead we need to use the "global" Private key

TO-DO:

• Make sure that Decryption Private Key" is actually used when there is value in the field
• move "Decryption private key" and "Decryption Certificate" fields under the "Want assertion signed" toggled and display them conditionally
• Document and make it clear (info text on the field?) that when "Decryption private key" is not filled the "Private key" will be used instead