Add a snmp_context argument to SNMP exporter

[ptodev]

PR Description

Adds a new config argument which was added to upstream recently

Notes to the Reviewer

I still haven't tested this. I hope to test it tomorrow with some help from @bastischubert

PR Checklist

• [x] CHANGELOG.md updated
• [x] Documentation added
• [x] Tests updated
• [x] Config converters updated