feat: Add validation for required fields in attendee form

[iamareebjamal]

Fixes #7068
Fixes #7067

Unblocks #7069

[houndci-bot]

Black would make changes.

[codecov]

Codecov Report

Merging #7070 into development will increase coverage by 0.14%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           development    #7070      +/-   ##
===============================================
+ Coverage        61.66%   61.81%   +0.14%     
===============================================
  Files              260      260              
  Lines            12915    12929      +14     
===============================================
+ Hits              7964     7992      +28     
+ Misses            4951     4937      -14     

Impacted Files	Coverage Δ
app/api/schema/attendees.py	100.00% <ø> (+1.69%)	⬆️
app/api/attendees.py	54.26% <100.00%> (+12.73%)	⬆️
app/models/custom_form.py	97.95% <100.00%> (+0.13%)	⬆️
app/api/helpers/permission_manager.py	35.78% <0.00%> (+1.05%)	⬆️

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