apply code review: refactor pfcp_endpoint API

Code review requested that the API should use functions instead of
direct access to a struct.

I have moved all user provided config to a separate struct
osmo_pfcp_endpoint_cfg, to be passed to osmo_pfcp_endpoint_create().
Halfway through those changes, I am not so certain whether that is what
reviewers had in mind. It makes sense from the point of view to keep nr
of arguments passed to osmo_pfcp_endpoint_create() small, and to allow
changing the user provided config without requiring a new
osmo_pfcp_endpoint_create2() API function. Though that again has ABI
compat problems, and makes no sense from the point of view that all
access should be done via API functions.

Personally I don't really agree with this change, which is probably the
reason why this patch ended up this way.

Related: SYS#5599
Change-Id: If80c35c6a942bf9593781b5a6bc28ba37323ce5e
---
M include/osmocom/pfcp/pfcp_endpoint.h
M src/libosmo-pfcp/pfcp_cp_peer.c
M src/libosmo-pfcp/pfcp_endpoint.c
3 files changed, 116 insertions(+), 55 deletions(-)