feat(openai): Make tiktoken encoding name configurable + tiktoken usage opt-in

[colin-sentry]

Tiktoken encodings sometimes make network requests, or do other stateful things. We can't guarantee what happens when you call tiktoken, so let's make it explicitly opt-in to users.

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[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 79.39%. Comparing base (301c4b8) to head (a1b35d8).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3289      +/-   ##
==========================================
- Coverage   79.40%   79.39%   -0.02%     
==========================================
  Files         132      132              
  Lines       14281    14276       -5     
  Branches     2999     3002       +3     
==========================================
- Hits        11340    11334       -6     
+ Misses       2094     2093       -1     
- Partials      847      849       +2     

Files	Coverage Δ
sentry_sdk/integrations/openai.py	77.98% <90.90%> (-0.55%)	⬇️
sentry_sdk/integrations/langchain.py	66.94% <61.53%> (-0.55%)	⬇️

[antonpirker]

Looks fine.
We need to update the docs for the integrations, though.