[Meshtastic] Add a mechanism for approximating mesh size

### Platform Cross-Platform ### Description The maximum number of active nodes is used in [default.cpp](https://github.com/meshtastic/firmware/blob/eb80d3141d2fe8deee3e13ec15f70bc19fcc9a94/src/mesh/Default.cpp#L38), but it is limited by the maximum number of nodes in the nodeDB. For very large meshes, this means that a node has no way to know whether they have an audience of 200 or 2000 nodes when they send a broadcast telemetry packet, for example. A more effective way to estimate the number of nodes is needed. Following discussion, this could take one of the following forms: 1. Bloom filter with a modest 12-bit hash, and a function to iterate periodically. 2. As above, but much larger hash to give a more accurate number 3. Other estimates using advanced techniques, e.g. [LogLog](https://en.wikipedia.org/wiki/Flajolet%E2%80%93Martin_algorithm) 4. Count a representative sample and apply a logical scaling function to give the estimated mesh size. Other important KPIs for the mesh