[Feature Request] [M] MSAL improves search of metadata in the federation metadata XML during WS-Trust flows.

[trwalke]

Is your feature request related to a problem? Please describe.
Continuation of #2152

Describe the solution you'd like
MSAL should search for the "wsdl:definitions" node within the provided federation metadata document during WS-Trust instead of assuming it is always the top node. WHile ADFS provided by Microsoft will always follow this pattern, third party IDPs may not which can cause authentication errors.

MSAL should search for the node in the Federation metadata provided by either the endpoint or the WithFederationMetadata(string federationMetadata) api instead of assuming that the top node will always have the metadata.
This will enable third party IDPs to provide federation metadata in a slightly different format.

The location where this parsing occurs is here:

microsoft-authentication-library-for-dotnet/src/client/Microsoft.Identity.Client/WsTrust/MexDocument.cs

Line 82 in 0d53f3e

IEnumerable<XElement> policyElements = mexDocument.Elements().First().Elements(XmlNamespace.Wsp + "Policy");

here:

microsoft-authentication-library-for-dotnet/src/client/Microsoft.Identity.Client/WsTrust/MexDocument.cs

Line 138 in 0d53f3e

IEnumerable<XElement> bindingElements = mexDocument.Elements().First().Elements(XmlNamespace.Wsdl + "binding");

and here:

microsoft-authentication-library-for-dotnet/src/client/Microsoft.Identity.Client/WsTrust/MexDocument.cs

Line 197 in 0d53f3e

XElement serviceElement = mexDocument.Elements().First().Elements(XmlNamespace.Wsdl + "service").First();

Alternatives
MSAL can provide an api that enables developers to provide the parent node of the XML node that contains the required "wsdl:definitions". If the required node is the top node this api should not be used as MSAL already searches the top node by default.

[trwalke]

There is still an ongoing discussion on whether or not MSAL should provide the api for this or do it internally in Email