[chore] [exporterhelper] Move to span context marshaling as a message

[dmitryax]

Resolves #13220 (comment)

Benchmarks:

goos: darwin
goarch: arm64
pkg: go.opentelemetry.io/collector/pdata/xpdata/request
cpu: Apple M1 Max

before:
BenchmarkEncodeDecodeContext-10    	  528876	      2331 ns/op	    1200 B/op	      28 allocs/op

after:
BenchmarkEncodeDecodeContext-10    	  559353	      1832 ns/op	     864 B/op	      16 allocs/op

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.50%. Comparing base (c779925) to head (3bf8428).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13230      +/-   ##
==========================================
- Coverage   91.51%   91.50%   -0.02%     
==========================================
  Files         522      522              
  Lines       28980    28988       +8     
==========================================
+ Hits        26522    26525       +3     
- Misses       1940     1942       +2     
- Partials      518      521       +3     

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