Cosmos DB: Add AAD authentication

[ealsur]

This PR adds support for azcore.TokenCredential as authentication method to the Cosmos DB SDK.

How to create a client with AAD

Using the default credential:

cred, err := azidentity.NewDefaultAzureCredential(nil)
if err != nil {
	panic(err)
}

client, err := azcosmos.NewClient(endpoint, cred, nil)
if err != nil {
	panic(err)
}

Using a client secret:

cred, err := azidentity.NewClientSecretCredential("tenantId", "clientId", "clientSecret")
if err != nil {
	panic(err)
}

client, err := azcosmos.NewClient(endpoint, cred, nil)
if err != nil {
	panic(err)
}

API changes

There is only a single API change, which adds the new client constructor:

func NewClient(endpoint string, cred azcore.TokenCredential, o *ClientOptions) (*Client, error)

APIView https://apiview.dev/Assemblies/Review/ca456a2f7ed64556b3fb57ecebc0b9d2/cc654e57f8044ed8a14c5dbf421ab49b?diffRevisionId=54a33d355ba146cb8e595448aa039540&doc=False&diffOnly=True

(There are other bogus differences due to reordering of existing properties)

Closes #16219