feat: Add SENTRY_SPOTLIGHT env variable support

[BYK]

Allows setting Spotlight through $SENTRY_SPOTLIGHT env variable.

Part of getsentry/spotlight#482

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 79.73%. Comparing base (4858996) to head (bb67772).

Files	Patch %	Lines
sentry_sdk/client.py	50.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3443      +/-   ##
==========================================
- Coverage   79.80%   79.73%   -0.08%     
==========================================
  Files         134      134              
  Lines       14522    14530       +8     
  Branches     3056     3059       +3     
==========================================
- Hits        11589    11585       -4     
- Misses       2097     2112      +15     
+ Partials      836      833       -3     

Files	Coverage Δ
sentry_sdk/client.py	79.25% <50.00%> (-0.56%)	⬇️

... and 2 files with indirect coverage changes

[sentrivana]

@BYK Thanks for this! Can you please also add a test to https://github.com/getsentry/sentry-python/blob/master/tests/test_spotlight.py?

[BYK]

Will add tests and then will request a re-review.

[BYK]

Finally, ready for the final round

[sentrivana]

Thank you @BYK!