mqtt: Improve handling of multiple PDU parsing

[jlucovsky]

Issue: 6592

Link to redmine ticket: 6592

Describe changes:

• Parse PDU instead of entire stream

Provide values to any of the below to override the defaults.

To use a pull request use a branch name like pr/N where N is the
pull request number.

Alternatively, SV_BRANCH may also be a link to an
OISF/suricata-verify pull-request.

SV_REPO=
SV_BRANCH=
SU_REPO=
SU_BRANCH=
LIBHTP_REPO=
LIBHTP_BRANCH=

[catenacyber]

Could you have a SV test ? Maybe adding bsize To the current mqtt-frames test ?

[suricata-qa]

Information: QA ran without warnings.

Pipeline 17328

[catenacyber]

This should not be current.len() as current may hold multiple PDUs, but rather start.len() - rem.len()

[catenacyber]

CI needs to be green as well

[jlucovsky]

Continued in #10236