[baggage] validate key and values during baggage injection/extraction

[lachmatt]

Fixes #5479 Design discussion issue #

Changes

• Baggage item key is no longer encoded and decoded - it is validated against token requirement from the spec instead
• Baggage item value is validated during extraction
• Only invalid baggage items are rejected during injection
• All of the baggage items are rejected during extraction if any of them is invalid (more strict than injection)

NOTE: Amount of allocations (e.g in extract part) can be reduced, I'd prefer to address it in a separate PR.

Merge requirement checklist

• [x] CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• [x] Unit tests added/updated
• [x] Appropriate CHANGELOG.md files updated for non-trivial changes
• [ ] Changes in public API reviewed (if applicable)