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.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information