NGAP: Wrong S-AMBR

Issue

• I have seen that for NGAP the S-AMBR is calculated wrongly, there are some edge cases where it is not correct.
• I have fixed the AMBR calculation for NAS and Lukas has fixed the GBR calculation for NGAP

@phine_lukas I suggest you have a look and integrate it into your GBR bitrate fix (The bitrate logic is the same between GBR and AMBR)

I think the problem is in this function in smf_context.cpp:

void smf_context::get_session_ambr(
    Ngap_PDUSessionAggregateMaximumBitRate_t& session_ambr,
    const snssai_t& snssai, const std::string& dnn);