Add extensibility to CredentialDescription to support custom signed assertion providers

[JoshLozensky]

This proposal pairs with an issue of the same name in the Microsoft.Identity.Abstractions repository where there are more details for the overall design.

The changes currently needed in Id Web to support this work are below:

Update the Microsoft.Identity.Web Json schema here to take into account the new CredentialSource value, and the new properties.

• Add the new CredentialSource value (CustomSignedAssertion) to the ClientCredentialSource
• Add the behavior for the new CredentialSource value after this block, and getting inspired by this block. For the part of the schema related to `CustomSignedAssertionProviderData, get inspiration from what was done in ExtraParameters in the AcquireTokenOptions