[Feature Request] Support a credential description for auto decrypt keys

[jmprieur]

Be able to support the following configuration:

            {
                "TokenDecryptionCredentials": [
                {
                    "SourceType": "AutoDecryptKeys",
                    "DecryptKeysApplicationTenant": "mytenant.onmicrosoftonline.com"
                    "DecryptKeysProtocol" : "bearer"
                }]
            }

This credential does not affect the container or ReferenceOrValue properties which should not be set and should return null for the soruce type "AutoDecryptKeys".

[jmprieur]

@RojaEnnam:
After discussion with @jennyf19, @GeoK and Keegan:
We're not going to have the application tenant and the protocol part of this credential, because we would actually need all the properties of AuthorizationHeaderProviderOptions. For instance if we need to use the Pop protocol, there is more that needs to be provided, in addition to the tenant (pop key, URL, etc ...)

Therefore.

• We'll add the source type AutoDecryptKeys.
• we could add an AuthorizationHeaderProviderOptions property here,

But I wonder if a better approach might not be to have a default property at the top Level, in MicrosoftIdentityApplicationOptions

/// <summary>
/// Default parameter used when the application needs to call a downstream API
/// on behalf of itself. 
/// In particular it describes the tenant to be used by default in the case of multi-tenant applications, but
/// can also describe the protocol used (Pop) and its parameters
/// </summary>
AuthorizationHeaderProviderOptions DefaultAppTokenOptions;

"DefaultDaemonScenarioOptions": {
    "AcquireTokenOptions": {
        "Tenant": "<tenant-id>"
        }
    }