Implement Application service mode for Matrix <-> VK bridge

[MurzNN]

Current bot-based bridge is good, because can be used without own Matrix server.

But for homeserver owners will be good to have mode with full implementation of bridge via application service, that emulate VK users as Matrix virtual users, that will look much native on Matrix side, that current bot-based bridging.

For implement Application service you can reuse https://github.com/Cadair/python-appservice-framework library.

[MurzNN]

Here https://github.com/tulir/mautrix-python is more fresh version of Python library for AS