gsup_client: Add new APIs to avoid users accessing struct fields

This is a first step towards changing gsup_client implementation to use
an osmo_stream_cli instead of libosmo-abis' ipa_client_conn.
The libosmo-abis' ipa_client_conn will eventually be deprecated together
with all IPA related code in libosmo-abis.

In order to be able to make the implementation change, we first need to
make sure no users are using the struct fields of gsup_client (this
patch); 2nd step will be making the struct private by moving it to a
private header; 3rd step will be changing the implementation to use
osmo_stream.

Related: OS#5896
Change-Id: I401af83232022f1c141eef1f428cbe206a8aaaa2
---
M TODO-RELEASE
M include/osmocom/gsupclient/gsup_client.h
M src/gsupclient/gsup_client.c
M src/gsupclient/gsup_client_mux.c
M src/remote_hlr.c
5 files changed, 59 insertions(+), 8 deletions(-)