MINOR: log: add dont-parse-log and assume-rfc6587-ntf options

MINOR: log: add dont-parse-log and assume-rfc6587-ntf options

This commit introduces the dont-parse-log option to disable log message
parsing, allowing raw log data to be forwarded without modification.

Also, it adds the assume-rfc6587-ntf option to frame log messages
using only non-transparent framing as per RFC 6587. This avoids
missparsing in certain cases (mainly with non RFC compliant messages).

The documentation is updated to include details on the new options and
their intended use cases.

This feature was discussed in GH #2856