Add exported functions to the snowflake client library to allow calling
programs to define and set their own custom broker rendezvous methods.

So the assignment of proxies is based on the load. The number of clients
is ronded down to 8. Existing proxies that doesn't report the number
of clients will be distributed equaly to new proxies until they get 8
clients, that is okish as the existing proxies do have a maximum
capacity of 10.

Fixes #40048

To be used by the debian package (#19409)

As we now partition proxies by NAT type, our stats are more useful if they
capture how many proxies of each type we have, and information on
whether we have enough proxies of the right NAT type for our clients.
This change adds proxy counts by NAT type and binned counts of denied clients by NAT type.

Add broker messaging specification with endpoints for clients and
proxies.

Added a folder to hold snowflake specifications. This folder starts with
a file containing a partial broker spec that focuses on the metrics
reporting spec for CollecTor at the moment.