chore: byte index to read singularEvent, propagate payload func to processor pipeline

[Sidddddarth]

Description

Use byte index to get event payload and propagate same to processor pipeline steps - using sync.OnceValue instead of go-kit/memoize so it can be propagated to other steps. This can be used in setting report payloads or settings event payloads to the error db.

Also updated tests to remove cases where processor picked up jobs from gateway that had multiple events - this hasn't been the case for a long time now.

Linear Ticket

Resolves PIPE-1717

Security

• The code changed/added as part of this pull request won't create any security issues with how the software is being used.

[codecov]

Codecov Report

Attention: Patch coverage is 85.07463% with 10 lines in your changes missing coverage. Please review.

Project coverage is 73.54%. Comparing base (f908e21) to head (f9498ae).
Report is 30 commits behind head on master.

Files with missing lines	Patch %	Lines
processor/processor.go	84.84%	9 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5314      +/-   ##
==========================================
- Coverage   73.57%   73.54%   -0.03%     
==========================================
  Files         424      424              
  Lines       59935    59961      +26     
==========================================
+ Hits        44095    44098       +3     
- Misses      13370    13397      +27     
+ Partials     2470     2466       -4     

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[github-actions]

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