feat: Make per_file rule support severities

[furtib]

Development halted until issues with integration are ironed out. #199
Why:
We want complete feature parity between the monolithic and the per_file rule.
The per_file rule currently does not support the severities attribute.

The per_file rule should behave exactly the same during parsing, and when checking the severities, any difference between the two should be considered a bug, not a feature.
That's why, instead of rewriting the logic for parsing and checking severities, I have reused the well-tested monolithic implementation.

What:

• Refactored necessary functions out of the codechecker script into a shared common library.
• Created a parse step for the per_file rule, reusing the one used for the monolithic version.
• Created a test step for the per_file rule, reusing the severity-checking function of the monolithic rule.

Addresses:
Fixes: #309
Fixes: #279

Depends on: #311