Allow minWireVersion/maxWireVersion fields in heartbeat responses to be configured

mongobetween currently responds to heartbeats (periodic `isMaster` requests) with `minWireVersion=0` and `maxWireVersion=7` in order to present as a 4.0 server. To be more flexible, users should be able to configure these via command line parameters and/or environmental variables. This would allow mongobetween to support multiple server versions and would also make server upgrades easier because no code changes would be required to the proxy.

No files, tests, or entry points are named in the issue. Start by locating mongobetween's command-line and environment configuration, then find the heartbeat response that sets minWireVersion and maxWireVersion; done means both values can be configured without code changes and heartbeat behavior reflects those settings.

Written by the indexing model from the issue text.