In AadIssuerValidator, make the config manager an injectable singleton

[pmaytak]

Currently in AadIssuerValidator, ConfigurationManager is a private static variable.

microsoft-identity-web/src/Microsoft.Identity.Web/Resource/AadIssuerValidator.cs

Line 25 in d33ab13

private static readonly ConfigurationManager<IssuerMetadata> s_configManager = new ConfigurationManager<IssuerMetadata>(Constants.AzureADIssuerMetadataUrl, new IssuerConfigurationRetriever());

Investigate to possibly make it an injectable singleton, so it's easier to fake/replace.

[Tratcher]

Compare to the options pattern AspNet uses that allows you to provide a backchannel and/or a configuration manager:
https://github.com/dotnet/aspnetcore/blob/3fdbc2e90d9762324ef3809098c9ca25b0cb3284/src/Security/Authentication/OpenIdConnect/src/OpenIdConnectPostConfigureOptions.cs#L67-L105

[jennyf19]

Included in 1.2.0 release.