fix(baggage): Update baggage parsing and encoding in vendored otel package

[tonyo]

These changes to baggage.go fix a few issues that are happening now in violation of the baggage spec (or at least my understanding of it):

1. Baggage string value one%20two should be properly parsed as "one two"
2. Baggage string value one+two should be parsed as "one+two"
3. Go string value "one two" should be encoded as one%20two (percent encoding), and NOT as one+two (URL query encoding).
4. Go string value "1=1" might be encoded as 1=1 (the spec says: "Note, value MAY contain any number of the equal sign (=) characters. Parsers MUST NOT assume that the equal sign is only used to separate key and value."). 1%3D1 is also valid, but to simplify the implementation we're not doing it.

Notes

• The original baggage_test.go was copied from opentelemetry-go, commit 5e8eb855bf3c6507715edd12ded5c6a950dd6104.
• Case number 2 is not fully fixed right now: baggage string value one+two will be parsed as "one two", and not as "one+two". To fix that, url.QueryUnescape() should be replaced by a function with the logic compatible with percentEncodeValue(), where the plus (and other allowed characters) are not treated as something special.

Fixes #554.

[codecov]

Codecov Report

Base: 74.72% // Head: 79.13% // Increases project coverage by +4.40% 🎉

Coverage data is based on head (9efd2cd) compared to base (ef3a838).
Patch coverage: 89.28% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #568      +/-   ##
==========================================
+ Coverage   74.72%   79.13%   +4.40%     
==========================================
  Files          38       38              
  Lines        3838     3853      +15     
==========================================
+ Hits         2868     3049     +181     
+ Misses        850      703     -147     
+ Partials      120      101      -19     

Impacted Files	Coverage Δ
internal/otel/baggage/baggage.go	97.54% <89.28%> (+53.49%)	⬆️

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[tonyo]

testify is currently used only in baggage_test.go, and also used by a bunch of dependencies.

[cleptric]

Nice! Maybe we can give it another try by opening this as a PR on the Otel repo.