Enabled RS256 signature for JWT access tokens

[fabionardelli]

Proposed changes

Modified generateJwtToken method in /models/model_oauth_server.js to allow signing the JWT access token with RS256 (asymmetric) algorithm when set through config.oidc.jwt_algorithm attribute or IDM_OIDC_JWT_ALGORITHM env variable.
This allows for an easier integration with common OAuth 2.0 libraries such as Spring Security OAuth 2.0 Resource Server.

Closes #357

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