Change Signal/SignalBackend args

[evalott100]

We should alter the kwargs of Signal/SignalBackend such that

• Signal takes an optional datatype in __init__ and do some validation on it
• EpicsSignalBackend takes datatype in __init__
• SignalBackend.connect now takes datatype instead
• On both Signal and SignalBackend __init__ and connect arg to kwarg inference is prevented

# Signal
    def __init__(
        self,
        *,
        backend: Optional[SignalBackend[T]] = None,
        datatype: Optional[SignalDataType] = None,
        timeout: Optional[float] = DEFAULT_TIMEOUT,
        name: str = "",
    ) -> None:
        #: If set, then the datatype of the value is guaranteed to be this type
        self.datatype = datatype
        ....

    async def connect(
        self,
        *,
        mock=False,
        timeout=DEFAULT_TIMEOUT,
        force_reconnect: bool = False,
        backend: Optional[SignalBackend[T]] = None,
    ):
        ....

# Signal backend

@abstractmethod
async def connect(
    self,
    *,
    datatype: Optional[type] = None,
    timeout: float = DEFAULT_TIMEOUT
):
    ....

Part of this change will be getting rid of conversion in mock signal backend.

Originally posted by @coretl in #310 (comment)