feat: add billing info mandatory field to event model

[abhinavk96]

Fixes #6268

Short description of what this resolves:

Adds a billing info mandatory field to the event model

[abhinavk96]

@iamareebjamal Fixed, please review again. Thanks!

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (development@f2773e5). Click here to learn what that means.
The diff coverage is 100%.

@@              Coverage Diff               @@
##             development    #6269   +/-   ##
==============================================
  Coverage               ?   65.78%           
==============================================
  Files                  ?      286           
  Lines                  ?    14519           
  Branches               ?        0           
==============================================
  Hits                   ?     9552           
  Misses                 ?     4967           
  Partials               ?        0

Impacted Files	Coverage Δ
app/api/schema/events.py	91.79% <100%> (ø)
app/models/event.py	79.19% <100%> (ø)

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