Support brotli & zstd compression

[ajashton]

Full PMTiles spec compliance requires supporting brotli & zstd compression.

• Use system-provided libraries/headers if available, but make it possible to build without them.
• Support brotli & zstd compression for MBTiles as well? This is not spec-compliant, but such tilesets may exist in the wild. If autodetection does not add too much overhead we might as well.

[ajashton]

Seems like low priority for now - neither brotli or zstd are currently supported by any of the PMTiles makers I am aware of (Tippecanoe, tilemaker, go-pmtiles).