[API] Parse baggage value as spec compliant

[ThomsonTan]

Fixes #3754

Changes

Please provide a brief description of the changes here.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.94%. Comparing base (b568e71) to head (d4bf0e4).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@ ## main #3758 +/- ## ========================================== + Coverage 89.93% 89.94% +0.01%  ========================================== Files 225 225 Lines 7158 7163 +5 ========================================== + Hits 6437 6442 +5  Misses 721 721 

Files with missing lines	Coverage Δ
api/include/opentelemetry/baggage/baggage.h	98.20% <100.00%> (+0.91%)	⬆️

... and 4 files with indirect coverage changes

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[marcalff]

LGTM, with minor cleanup.

[marcalff]

Several comments:

1. Please add a comment that points to the spec: https://www.w3.org/TR/baggage/#definition, to explain what this is about.

2. Per the spec, a space (20) should be illegal. the test str[i] >= ' ' seems broken.

3. Consider using parenthesis and comments for better readability, as in

if ( (str[i] >= '!' ) /* 21 */ && (str[i] <= '~' ) /* 7E */ && (str[i] != '"') /* 22 */ && (str[i] != ',') /* 2C */ && (str[i] != ';') /* 3B */ && (str[i] != '\\')) /* 5C */