add ChannelType values for gRPC channels

[hokiegeek2]

Feature Description
Add ChannelType Enum values for gRPC Channel implementations as follows:

class ChannelType(Enum):
    """
    The ChannelType Enum specifies which Channel implementation is
    to be used for an Arkouda client deployment.
    """
    ZMQ = 'ZMQ'
    GRPC = 'GRPC'
    ASYNC_GRPC = 'ASYNC_GRPC'
    STREAMING_GRPC = 'STREAMING_GRPC'

    def __str__(self) -> str:
        """
        Overridden method returns value.
        """
        return self.value

    def __repr__(self) -> str:
        """
        Overridden method returns value.
        """
        return self.value