File system persistence for sessions

[Tyrrrz]

Closes #1096

[Tyrrrz]

Still missing support for special handling of paused sessions (will have to store additional data in the file) and DidCrash handler. But what do you think about the general direction @bruno-garcia?

[Tyrrrz]

@bruno-garcia some significant changes.
I had to change how the pause/resume works so that it's more encapsulated within session manager, which would allow it to take ownership of the file persistence logic. As a result, it now returns multiple updates from ResumeSession(...) back to Hub. Ugly, but I don't think there are better options since GlobalSessionManager does not encapsulate a reference to ISentryClient (although that wouldn't be a horrible idea maybe?).
Evaluating race conditions and thread contention is a nightmare now, too many variables. Part of me just wants to wrap everything in locks.
What do you think?

[codecov-commenter]

Codecov Report

Merging #1105 (c8dc470) into main (1b7e3c8) will decrease coverage by 0.32%.
The diff coverage is 72.30%.

@@            Coverage Diff             @@
##             main    #1105      +/-   ##
==========================================
- Coverage   80.97%   80.64%   -0.33%     
==========================================
  Files         195      197       +2     
  Lines        6422     6582     +160     
  Branches     1417     1452      +35     
==========================================
+ Hits         5200     5308     +108     
- Misses        761      803      +42     
- Partials      461      471      +10     

Impacted Files	Coverage Δ
src/Sentry/Internal/Hub.cs	61.61% <29.16%> (-7.74%)	⬇️
src/Sentry/Internal/Http/CachingTransport.cs	78.14% <66.66%> (-0.57%)	⬇️
src/Sentry/SentryOptionsExtensions.cs	71.42% <71.42%> (ø)
src/Sentry/GlobalSessionManager.cs	70.44% <74.43%> (+3.00%)	⬆️
src/Sentry/IJsonSerializable.cs	100.00% <100.00%> (ø)
src/Sentry/Internal/Json.cs	100.00% <100.00%> (ø)
src/Sentry/PersistedSessionUpdate.cs	100.00% <100.00%> (ø)
src/Sentry/SentryOptions.cs	89.24% <100.00%> (+0.06%)	⬆️
src/Sentry/SessionUpdate.cs	93.39% <100.00%> (+0.12%)	⬆️
... and 2 more

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[bruno-garcia]

Some small suggestions but LGTM