fix: add parentContext to onStart

[Flarna]

Which problem is this PR solving?

Fixes #2756

Short description of the changes

Add parentContext argument to onStart in SpanProcessors to allow to use it in a subclass without complains by typescript.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

local

Checklist:

• Followed the style guidelines of this project
• Unit tests have been added
• Documentation has been updated

[codecov]

Codecov Report

Merging #2757 (0abbfa0) into main (94a0747) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2757   +/-   ##
=======================================
  Coverage   93.27%   93.27%           
=======================================
  Files         159      159           
  Lines        5444     5444           
  Branches     1142     1142           
=======================================
  Hits         5078     5078           
  Misses        366      366           

Impacted Files	Coverage Δ
...dk-trace-base/src/export/BatchSpanProcessorBase.ts	94.38% <100.00%> (ø)
...y-sdk-trace-base/src/export/SimpleSpanProcessor.ts	91.30% <100.00%> (ø)