[Py] Retrieve External Pinned Maps

Hi everyone,

I was wondering if BCC's Python library supports retrieving a BPF map from the file system (AKA a pinned map). I am loading an XDP program in C using [libbpf](https://github.com/libbpf/libbpf/) and pinning maps to the file system.

I am using a higher-level library in Python to retrieve information and trying to update the BPF maps with this information.

I've searched the reference guide [here](https://github.com/iovisor/bcc/blob/master/docs/reference_guide.md#bcc-python) and wasn't able to find any way to do this. It appears to be possible in BCC C according to [here](https://github.com/iovisor/bcc/blob/master/docs/reference_guide.md#pinned-maps), though. I also believe this is similar to the following.

https://github.com/iovisor/bcc/issues/2223

If this is currently not supported, is there any plans to add support in the future?

Any help is highly appreciated and thank you!

Start with the BCC Python and pinned-maps sections of docs/reference_guide.md, then compare the existing C support and related issue #2223. Determine the Python-facing entry point needed to retrieve a filesystem-pinned map and update it; done means the documented API works with maps pinned by an XDP program.

Written by the indexing model from the issue text.