feat: add filter for junos perfmon events

[zyphermonkey]

create filter to capture junos events.

examples

PERF_MON: RTPERF_CPU_UTIL_MAX: FPC 0 PIC 0 CPU Utilization greater than 99, expect packet loss

PERF_MON: RTPERF_CPU_THRESHOLD_EXCEEDED: FPC 0 PIC 0 CPU utilization exceeds threshold, current value = 99

[mstopa-splunk]

@zyphermonkey thank you for the PR and I'm sorry for the delayed review.

To make us able to merge please:

1. Include changes in both:

package/etc/conf.d/conflib/syslog/app-syslog-juniper_junos_unstructured.conf
package/lite/etc/addons/juniper/app-syslog-juniper_junos_unstructured.conf

2. Add the new sourcetype to docs/sources/vendor/Juniper/junos.md

3. Add a test case to tests/test_juniper_junos_rfc3164.py