proxy lib - be able to configure the proxy type

The snowflake proxy library is being used by more clients than our standalone proxy. Library users should be able to set the proxy type that will be reported to the broker.

Currently the proxy type is hardcoded: https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/blob/main/proxy/lib/snowflake.go#L202

A use case for it now will be to have a different type in orbot, so we know how many snowflakes are provided by orbot users in comparison to other users.

We should take into account that currently the broker has a hardcoded list of proxy types and the rest is treated as 'unknown'. This was motivated by having a lot of requests with estrange proxy types (#40089 (closed)). I guess we can extend the proxy type list for the mayor types we know off or we could do some simple validation of what kind of proxy types are meaningful.