Support MSC pooling
Harald wrote in https://gerrit.osmocom.org/c/osmo-bsc-nat/+/27473:
Please note that we cannot assume just one MSC, but have to consider the situation of MSC pooling, too. Most if not all real netwokrs use MSC pooling, and we support it in the BSC as well just as we support SGSN pooling in the PCU and gbproxy.
any vty configuration regarding pooling should look as much as possible identical to how it is done in other places (gbproxy, for example)