### Description

Attempting to extract SIP message payload using Python3 KEMI fails with exception if the payload contains 8-bit ISO 8859-1 encoded characters.

According to Kamailio docs, KEMI functions return either string (which for Python is UTF-8), integer or null, which means 8-bit/binary payloads cannot be handled at all? This seems to go against the [SIP spec](https://www.rfc-editor.org/rfc/rfc5621.html#section-3.2), which states that SIP is 8-bit safe:

SIP messages can carry binary message bodies such as legacy signalling objects [[RFC3204](https://www.rfc-editor.org/rfc/rfc3204)]. SIP proxy servers are 8-bit safe. That is, they are able to handle binary bodies. Therefore, there is no need to use encodings such as base64 to transport binary bodies in SIP messages. Consequently, UAs SHOULD use the binary transfer encoding [[RFC4289](https://www.rfc-editor.org/rfc/rfc4289)] for all payloads in SIP, including binary payloads. The only case where a UA MAY use a different encoding is when transferring application data between applications that only handle a different encoding (e.g., base64).

### Troubleshooting

#### Reproduction

Attempt to extract SIP message payload e.g. `KSR.pv.get("$msg(body)")` when the body has non-ASCII or UTF-8 characters, such as 'é'.

#### Debugging Data

#### Log Messages

(Proprietary specifics redacted)

``` Sep 14 22:40:09 localhost kamailio[772]: 2(65) CRITICAL: {1 2 MESSAGE Oy9m3MMLFwBKK_EQC-WS1g..} <core> [core/kemi.c:136]: sr_kemi_core_crit(): Exception during routing: Traceback (most recent call last): Sep 14 22:40:09 localhost kamailio[772]: File "xxx.py", line xxx, in xxx Sep 14 22:40:09 localhost kamailio[772]: body = KSR.pv.get("$msg(body)") Sep 14 22:40:09 localhost kamailio[772]: UnicodeDecodeError: 'utf-8' codec can't decode byte 0xe9 in position 0: invalid continuation byte ```

#### SIP Traffic

Full SIP message contains proprietary information. SIP message body for demonstration in above log is a single character 'é'). An otherwise identical message with body 'e' will instead decode successfully.

### Possible Solutions

### Additional Information

* **Kamailio Version** - output of `kamailio -v`

``` version: kamailio 5.6.2 (x86_64/linux) flags: USE_TCP, USE_TLS, USE_SCTP, TLS_HOOKS, USE_RAW_SOCKS, DISABLE_NAGLE, USE_MCAST, DNS_IP_HACK, SHM_MMAP, PKG_MALLOC, Q_MALLOC, F_MALLOC, TLSF_MALLOC, DBG_SR_MEMORY, USE_FUTEX, FAST_LOCK-ADAPTIVE_WAIT, USE_DNS_CACHE, USE_DNS_FAILOVER, USE_NAPTR, USE_DST_BLOCKLIST, HAVE_RESOLV_RES, TLS_PTHREAD_MUTEX_SHARED ADAPTIVE_WAIT_LOOPS 1024, MAX_RECV_BUFFER_SIZE 262144, MAX_URI_SIZE 1024, BUF_SIZE 65535, DEFAULT PKG_SIZE 8MB poll method support: poll, epoll_lt, epoll_et, sigio_rt, select. id: unknown compiled with gcc 9.4.0 ```

* **Operating System**:

``` NAME="Ubuntu" VERSION="20.04.6 LTS (Focal Fossa)" ID=ubuntu ID_LIKE=debian PRETTY_NAME="Ubuntu 20.04.6 LTS" VERSION_ID="20.04" HOME_URL="https://www.ubuntu.com/" SUPPORT_URL="https://help.ubuntu.com/" BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/" PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy" VERSION_CODENAME=focal UBUNTU_CODENAME=focal

Linux 31e2ff2f5338 5.15.0-83-generic #92-Ubuntu SMP Mon Aug 14 09:30:42 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux ```