Add Keys() method to propagation.TextMapCarrier

[punya]

...and propagation.HeaderCarrier to adapt http.Header to this interface.

Fixes #1493.

[codecov]

Codecov Report

Merging #1544 (1e94882) into main (0b1a1c7) will decrease coverage by 0.0%.
The diff coverage is 58.3%.

@@           Coverage Diff           @@
##            main   #1544     +/-   ##
=======================================
- Coverage   78.0%   78.0%   -0.1%     
=======================================
  Files        127     127             
  Lines       6590    6600     +10     
=======================================
+ Hits        5146    5153      +7     
- Misses      1200    1203      +3     
  Partials     244     244             

Impacted Files	Coverage Δ
bridge/opentracing/bridge.go	47.8% <0.0%> (ø)
propagation/propagation.go	78.5% <40.0%> (-21.5%)	⬇️
oteltest/text_map_propagator.go	76.2% <100.0%> (+1.5%)	⬆️