Move MP traget_kpi from facade to MixedPrecisionQuantizationConfig

[ofirgo]

Pull Request Description:

This change solves two problems:

• Usability: now it's clear that in order to enable mixed precision quantization, the user should provide a mixed precision configuration inside the core config (we still check that the target KPI is not None, but this is not part of "mixed_precision_enabled" condition).
• API clarity: target_kpi is not a part of PQT, GPTQ of QAT facades, and it was unreasonable to include it there, since it is only relevant for mixed precision (running target_kpi remains a part of running facade arguments).

Checklist before requesting a review:

• I set the appropriate labels on the pull request.
• I have added/updated the release note draft (if necessary).
• I have updated the documentation to reflect my changes (if necessary).
• All function and files are well documented.
• All function and classes have type hints.
• There is a licenses in all file.
• The function and variable names are informative.
• I have checked for code duplications.
• I have added new unittest (if necessary).

[elad-c]

It's about time