Integrate GDB pretty printers for server vocabulary types

We should merge (and greatly extend) the GDB pretty printers from https://github.com/redbeard0531/mongo_gdb into the server repository to make them generally available to developers.

We should probably add them under etc/gdb.

We should probably also write a top level project-specific .gdbinit so that the extensions can be auto-loaded:

https://sourceware.org/gdb/current/onlinedocs/gdb/Auto_002dloading.html#Auto_002dloading