feat: add support for unix sockets to TCPTransport in erpc_python

[brussee]

• feat: add support for unix sockets to TCPTransport in erpc_python
• chore: organize imports

[JanKomarekNXP]

Hi, we reviewed your PR and really like the idea of adding a UNIX socket to the transport layers. After discussing with @MichalPrincNXP, we concluded that since UNIX sockets do not use TCP, it would be more appropriate to create a new transport layer.

Below is the code that incorporates your changes into a new UnixSocketTransport layer. Could you please test it and update the PR accordingly?

Note that the code has been slightly modified, as we are also preparing an update to the Python implementation.

# Import
from typing import Optional

# Transport
class UnixSocketTransport(FramedTransport):
    def __init__(self, unix_socket: str, is_server: bool):
        super(UnixSocketTransport, self).__init__()
        self._unix_socket = unix_socket
        self._is_server = is_server
        self._sock: Optional[socket.socket] = None

        if self._is_server:
            self._server_thread = threading.Thread(target=self._serve)
            self._server_thread.daemon = True
            self._server_thread.start()
            self._server_sock_event_start = threading.Event()
        else:
            self._sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
            self._sock.connect(self._unix_socket)

    def _serve(self):
        s = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
        s.setblocking(True)
        s.bind(self._unix_socket)

        while True:
            self._sock, _ = s.accept()
            self._server_sock_event_start.set()

    def close(self):
        if self._is_server:
            self._server_sock_event_start.clear()

        if self._sock:
            self._sock.shutdown(socket.SHUT_RDWR)
            self._sock.close()
            self._sock = None

    def _base_send(self, data: bytes):
        if self._is_server:
            self._server_sock_event_start.wait()
        if self._sock:
            self._sock.sendall(data)

    def _base_receive(self, count: int) -> bytes:
        if self._is_server:
            self._server_sock_event_start.wait()

        if self._sock:
            remaining = count
            result = bytes()
            while remaining:
                data = self._sock.recv(remaining)
                if len(data) == 0:
                    self.close()
                    raise ConnectionClosed()
                result += data
                remaining -= len(data)
            return result

        return bytes()