[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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[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 */