ref(init): Move sentry_sdk.init out of hub.py

[szokeasaurusrex]

Now that the Hub-based API is deprecated, sentry_sdk.init should no longer be in hub.py. Since it is kind of its own thing, it makes sense to implement init in its own file.

Closes #3233

[szokeasaurusrex]

This comment is inaccurate, since we actually use _InitGuard in the class definition. However, since I am just copying this code over from hub for now, I think it is out of scope to fix here. Perhaps, we can consider fixing in a separate PR later.

[sl0thentr0py]

I also don't really know what the purpose of the _InitGuard is since we never tell people to do the following anywhere, so this is just vague unadvertised functionality.

with sentry_sdk.init():
    pass

[szokeasaurusrex]

Indeed, perhaps we should explicitly deprecate this context manager functionality and remove in the next major, in that case? Or is this a potentially valuable feature we should advertise more?

[sl0thentr0py]

init is generally one per process anyway. In case people want DSN isolation, multiple clients is the way to go now. So I really don't see the use case for this.
Let's deprecate and remove.

[szokeasaurusrex]

@sl0thentr0py, I opened an issue to make this change, since I think it belongs in a separate PR. Would appreciate an approval here if everything else looks good to you.

[codecov]

Codecov Report

Attention: Patch coverage is 64.28571% with 10 lines in your changes missing coverage. Please review.

Project coverage is 79.43%. Comparing base (06d5da1) to head (d5920bc).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3276      +/-   ##
==========================================
- Coverage   79.44%   79.43%   -0.01%     
==========================================
  Files         132      133       +1     
  Lines       14278    14282       +4     
  Branches     2999     3000       +1     
==========================================
+ Hits        11343    11345       +2     
- Misses       2088     2089       +1     
- Partials      847      848       +1     

Files	Coverage Δ
sentry_sdk/__init__.py	100.00% <100.00%> (ø)
sentry_sdk/hub.py	53.76% <ø> (-1.19%)	⬇️
sentry_sdk/_init_implementation.py	61.53% <61.53%> (ø)

... and 2 files with indirect coverage changes