Add support for the special common/organizations/consumers Azure AD tenants

[ntherning]

When not targeting a specific Azure AD tenant (specified by a tenant GUID in the discovery document URL) but rather one of the "common", "organizations" or "consumers" multi-tenant aliases (see 1), discovery document parsing and ID token validation require a few extra steps:

• The discovery document's "issuer" value contains the special placeholder "{tenantid}". As '{' and '}' are invalid characters in URLs, AppAuth has to URL encode these characters before the issuer URL can be parsed by NSURL in OIDServiceDiscovery.m.
• The same "{tenantid}" placeholder needs to be replaced with the actual tenant ID of the authenticated user, from the "tid" claim (see 2) of the ID token, before ID token validation is performed in OIDAuthorizationService.m.

1: https://docs.microsoft.com/en-us/azure/active-directory/develop/v2-protocols-oidc#fetch-the-openid-connect-metadata-document 2: https://docs.microsoft.com/en-us/azure/active-directory/develop/id-tokens#payload-claims

[ntherning]

For full Azure AD support in AppAuth-iOS PR #710 also has to be merged. See my comment on that issue: https://github.com/openid/AppAuth-iOS/pull/710#issuecomment-1172169490

[ntherning]

I have signed the CLA.

[AristideVB]

any updates on this PR @ntherning

[ntherning]

Still waiting for someone to review this PR. I signed the CLA almost 2 years ago. Just now I rebased it onto master. Perhaps @mdmathias who have been committing to this repo lately can review it? As I mentioned in a previous comment, the #710 PR has to be merged too to have Azure working. In the Spamdrain app we have supported Azure OpenID Connect sign ins/ups for years using AppAuth-iOS patched with this PR (and #710) with great success.

[AristideVB]

Ok thank you @ntherning looking forward for the merge, great app by the way ! 🎉 🙂

Without using your branch do you know of a workaround to have https://login.microsoftonline.com/consumers/v2.0/.well-known/openid-configuration work ?

[ntherning]

@AristideVB I'm afraid I don't know how to make it work with Azure without #710 and #714. The Spamdrain fork at https://github.com/spamdrain/AppAuth-iOS has a branch which adds #710 and #714 on top of the 1.6.2 release. Here's how we use that in our app's Podfile:

pod 'AppAuth', :git => 'https://github.com/spamdrain/AppAuth-iOS.git', :branch => '1.6.2+azure'