[API] Baggage - Trim white spaces and check for semi colon which isn't allowed octet

[nabutabu]

Towards #6816, #5210

Changes

• Fixed BaggagePropagator to trim optional whitespace (OWS) around = separators
  when parsing the baggage header, as required by the
  W3C Baggage specification. This caused
  incompatibility with .NET 10's default W3C propagator, which emits headers with
  OWS (e.g. key = value), resulting in spaces being URL-encoded as + in
  outbound headers received by downstream services.
• Fixed BaggagePropagator to strip baggage properties (;metadata) from values
  when parsing the baggage header. Semicolon is not a valid baggage-octet per
  the W3C Baggage specification and must be interpreted as the start of a property,
  not part of the value.
  (#5210)

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
  I don't think this is non-trivial but let me know if I need to do this
• Changes in public API reviewed (if applicable)