Fix the definition of CustomCredential to match actual service response

[dargilco]

This PR does not introduce a breaking change in REST APIs. I fixes the TypeSpec to match the current behavior of the REST APIs, so ref-docs are accurate.

Details:

In my Foundry project I created a connection of type "Custom Keys", and provide a couple of public keys and a couple of secret keys, as shown here:

When I do a GET REST API to the /connections route to get the connection properties of this Foundry connection, this is the JSON response (as an example):

                      {
                            "name": "test_custom_connection",
                            "id": "/subscriptions/.../connections/test_custom_connection",
                            "type": "CustomKeys",
                            "target": "_",
                            "isDefault": true,
                            "credentials": {
                                "nameofprivatekey1": "PrivateKey1",
                                "nameofprivatekey2": "PrivateKey2",
                                "type": "CustomKeys"
                            },
                            "metadata": {
                                "NameOfPublicKey1": "PublicKey1",
                                "NameOfPublicKey2": "PublicKey2"
                            }
                        }

As you see the private (secret) keys are exposed directly in the "credentials" JSON element. Not under a new element "credentials.keys" as the TypeSpec says. A better design would have been to expose all secret keys under "credentials.keys" however since the REST API are already GA, we can't change that. So this PR updates the TypeSpec to match the service
behavior.

See also companion Python SDK PR: Azure/azure-sdk-for-python#42937

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API Change Check

APIView identified API level changes in this PR and created the following API reviews

Language	API Review for Package
TypeSpec	Azure.AI.Projects
Java	com.azure:azure-ai-projects