ref(transport): Improve event data category typing

[szokeasaurusrex]

Done to more clearly define event data categories, in preparation for https://github.com/getsentry/sentry-python/issues/3229.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79.35%. Comparing base (31efa62) to head (854b46c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3243   +/-   ##
=======================================
  Coverage   79.34%   79.35%           
=======================================
  Files         132      132           
  Lines       14227    14227           
  Branches     2987     2987           
=======================================
+ Hits        11289    11290    +1     
+ Misses       2093     2092    -1     
  Partials      845      845           

Files	Coverage Δ
sentry_sdk/transport.py	80.73% <50.00%> (+0.26%)	:arrow_up:
sentry_sdk/types.py	75.00% <66.66%> (+3.57%)	:arrow_up:

... and 2 files with indirect coverage changes