adding iterator interface and removing GetAll() method from attribute list

[varkey98]

Adding an iterator interface on AttributeList so that the iterator can be queried without creating an additional slice.
Continuation of this pr: #217
ref: https://github.com/hypertrace/goagent/pull/217/files#r1385540220

Checklist:

• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• Any dependent changes have been merged and published in downstream modules

[codecov]

Codecov Report

Merging #220 (17f0db7) into main (de9ef45) will increase coverage by 0.61%.
Report is 1 commits behind head on main.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##             main     #220      +/-   ##
==========================================
+ Coverage   58.94%   59.56%   +0.61%     
==========================================
  Files          55       55              
  Lines        2236     2248      +12     
==========================================
+ Hits         1318     1339      +21     
+ Misses        859      850       -9     
  Partials       59       59              

Files	Coverage Δ
instrumentation/opentelemetry/span.go	86.81% <40.00%> (-3.51%)	⬇️

... and 2 files with indirect coverage changes

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[varkey98]

@ryanericson @tim-mwangi Please merge this pr, I cant merge since I dont have write access