fix(openai): Lazy initialize tiktoken to avoid http at import time

[colin-sentry]

tiktoken.get_encoding makes a HTTP connection if the encoding doesn't already exist.

If a user has OpenAI, sentry, and tiktoken all installed, this line would make a HTTP request at import time.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.36%. Comparing base (ae034ab) to head (4b63cf0).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3287      +/-   ##
==========================================
- Coverage   79.41%   79.36%   -0.05%     
==========================================
  Files         132      132              
  Lines       14279    14281       +2     
  Branches     2998     2999       +1     
==========================================
- Hits        11339    11334       -5     
- Misses       2094     2096       +2     
- Partials      846      851       +5     

Files	Coverage Δ
sentry_sdk/integrations/openai.py	78.52% <100.00%> (+0.26%)	⬆️

... and 4 files with indirect coverage changes