Configurable Cache Control for Discovery Endpoints

[gnjack]

Adds feature requested for IdentityServer4 in IdentityServer/IdentityServer4#810 but this time for IdentityServer3

It would be nice if there was a supported way to define cache headers for the discovery endpoint. Right now, our resource servers assume that the jwk immediately expires if there is no cache header set.

Add DiscoveryOptions.ClientCacheInterval to set the maxage cache control header on the discovery endpoints. Defaults to one hour.

This gives clients a hint how often they should refresh their cached copy of the discovery document.

Feature implemented in a similar manner to IdentityServer4 IdentityServer/IdentityServer4@58e6c3a