Setting what token signing key to use per OAuth application

**Is your feature request related to a problem? Please describe.**
Defguard supports several types of signing algorithms for OAuth2. Some applications only support a single type of signing algorithm. For deployments with several OAuth2 applications an option to set which algorithm to use is very important for comparability with those apps and use cases.

**Describe the solution you'd like**
A drop down in the settings panel for an OAuth2 app that shows the possible signing algorithms.
The server would use that to decide which to use when that application communicates with Defguard.

Also setting the default signing algorithm in the env would be very useful.