Rationale: in order to add full SDP to the MNCC protocol (upcoming patch
I8c3b2de53ffae4ec3a66b9dabf308c290a2c999f), we need to parse and compose SDP
messages. Obviously, libosmo-mgcp-client already contains similar code, but
that is unfortunately heavily glued to the actual MGCP implementation. The
simplest solution is to create this separate implementation, copy-pasting from
the existing libosmo-mgcp-client code as is convenient.
This API is added here to probe whether it works well. When it does, the
intention is to "move it up" to osmo-mgw and overhaul the SDP parsing in our
MGCP client and MGCP server APIs using this same API.