[Tables] Add support for AzureNamedKeyCredential

[annatisch]

This should be in place of the current dict/str based implementation.
The only supported types for the credential parameter will now be AzureNamedKeyCredential, AzureSasKeyCredential or via connection string (with samples/readme etc updated to match).

The azure-core dependency version will also need to be updated to match.

[seankane-msft]

@annatisch should this be AzureKeyCredential ?

[annatisch]

No, AzureNamedKeyCredential
https://github.com/Azure/azure-sdk-for-python/blob/master/sdk/core/azure-core/azure/core/credentials.py#L117