core: add sip_parser_log core variable

[lazedo]

• Commit message has the format required by CONTRIBUTING guide
• Commits are split per component (core, individual modules, libs, utils, ...)
• Each component has a single commit (if not, squash them into one commit)
• No commits to README files for modules (changes must be done to docbook files
  in doc/ subfolder, the README file is autogenerated)

Type Of Change

• Small bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds new functionality)
• Breaking change (fix or feature that would change existing functionality)

Checklist:

• PR should be backported to stable branches
• Tested changes locally
• Related to issue #XXXX (replace XXXX with an open issue number)

Description

when handling non sip messages, parser errors are logged at core level which defaults to ERR.
since this may induce in error sysops or log scanners, sometimes the core level is set to debug just to omit these
messages from log, which is not good since other messages are omitted too.
this commit adds a new core variable to set the log level for message parsing errors.
this way, we can explicitly set the log level for message parsing while leaving the core level at ERR.