feat(opentelemetry-instrumentation-document-load): Add access to performance resource timing object for custom attributes

[pkanal]

Which problem is this PR solving?

Allows access to the PerformanceResourceTiming object for resourceFetch spans to add custom attributes to the resourceFetch span. This would allow for the ability to add extra span attributes to resourceFetch spans like some common resource timing metrics.

Short description of the changes

• Added a function _addCustomAttributesOnResourceSpan that replaces _addCustomAttributesOnSpan when resourceFetch spans are created.
• _addCustomAttributesOnResourceSpan takes an additional resource argument that passes PerformanceResourceTiming information for each resource to the applyCustomAttributesOnSpan callback.

[codecov]

Codecov Report

Merging #1529 (638c97a) into main (8499b16) will decrease coverage by 0.08%.
The diff coverage is 88.88%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1529      +/-   ##
==========================================
- Coverage   96.13%   96.06%   -0.08%     
==========================================
  Files          14       14              
  Lines         906      914       +8     
  Branches      197      199       +2     
==========================================
+ Hits          871      878       +7     
- Misses         35       36       +1     

Impacted Files	Coverage Δ
...strumentation-document-load/src/instrumentation.ts	98.13% <88.88%> (-0.86%)	⬇️

[pichlermarc]

Looks great 🙂
Just a few nits from my side :)