Add support for parsing bridge-distribution-request lines in Stem

From #18329 (moved), we added a new bridge-server-descriptor line: bridge-distribution-request, which, if set to "none" should tell BridgeDB not to distribute a bridge. This allows, for example, BridgeDB to avoid distributing the default Tor Browser bridges, and allows other bridges to not be distributed, but to still submit their usage statistics for Metrics.