Add Python 3 Binding Generation via Pybind11

* Created entry point packet/python3_module.cc to create
  bluetooth_packets_python3 Python module. Layer specific
  packets are submodules of this top layer module
* Define shim layer for Builder::Create() functions to
  use a std::shared_ptr instead of std::unique_ptr as
  Python does not support explicit ownership transfer
* Add py::keep_alive to make sure the shared_ptr does not
  get garbage collected until the returned object is garbage
  collected
* Redefine PacketView::Create() method as a constructor for packet view
  and throws value errors when packet is in valid so that
  users don't have to call IsValid()
* Redefine Builder::Serialize() method to return a Python
  list of bytes directly instead of using BitInserter.
  Python binding users need to manually concatenate
  sub-sections of generated packets to form a bigger packet
* Add build rules to generate and build the Python 3 library

Example:

>>> import bluetooth_packets_python3 as bp3
>>> write_ssp_enable_builder = bp3.hci_packets.WriteSimplePairingModeBuilder(bp3.hci_packets.Enable.ENABLED)
>>> write_ssp_enable_bytes = write_ssp_enable_builder.Serialize()
>>> list(map(lambda b : hex(b), write_ssp_enable_bytes))
['0x56', '0xc', '0x1', '0x1']
>>> write_ssp_enable_pkt_view = bp3.PacketViewLittleEndian(write_ssp_enable_bytes)
>>> write_ssp_enable_view = bp3.hci_packets.WriteSimplePairingModeView(bp3.hci_packets.SecurityCommandView(bp3.hci_packets.CommandPacketView(write_ssp_enable_pkt_view)))
>>> write_ssp_enable_view.GetOpCode()
OpCode.WRITE_SIMPLE_PAIRING_MODE
>>> write_ssp_enable_view.GetSimplePairingMode()
Enable.ENABLED

Bug: 143374372
Test: make and run locally
Change-Id: I8775c2d26a393aa5c1a417b7f845f7102fe4120e