Cloud Shell ManagedIdentitySource should throw if a user-assigned managed identity clientID or resourceID is supplied

[christothes]

If the Cloud Shell ManagedIdentitySource is detected via MSAL and a user-assigned managed identity clientID or resourceID is supplied, the ManagedIdentityCredential should throw.

The rationale for taking this minor breaking change is that CloudShell does not support specifying a clientID or ResourceID and the current behavior of silently falling back to attempting to use a system-assigned identity could be unexpected.

[ahsonkhan]

I suspect this is the wrong behavior (putting the client_id value into the request body):

azure-sdk-for-cpp/sdk/identity/azure-identity/src/managed_identity_source.cpp

Lines 268 to 271 in 4dd4b18

	if (!clientId.empty())
	{
	m_body = std::string("client_id=" + Url::Encode(clientId));
	}

[ahsonkhan]

Issues from other languages (for reference):
Azure/azure-sdk-for-go#23179
Azure/azure-sdk-for-java#41119
Azure/azure-sdk-for-python#36438
Azure/azure-sdk-for-js#30380