Federation Metadata fallback

[rpmansion]

Recently, we keep encountering an issue of not being able to load the federation metadata from their domain which will not load all of the idp configurations to the lookup dictionary.

I'm thinking if we can consider adding a fallback where, if it fails, the library will fetch from a local copy of the idp metadata configuration or a DI where a developer can implement their own codes to do this.

[SrxDevOps]

We may have a similar issue. We have Identity Server (Duende Enterprise) scaled out to 3 servers in an Azure web app. Occasionally, when deploying new code and swapping slots between staging -> production, 1 of the 3 servers will hang on startup at LoadMetadata=True. We can see the HTTP dependency call lasting 2+ minutes. Restarting the hung server usually results in a successful start.