Add document creation and tasks, fix timestamps

[VMelnalksnis]

Fixes #148, creates #150

Changes proposed in this pull request:

• Use OffsetDateTime instead of Instant in response models, since the format depends on config
• Add additional methods to the client that were needed for testing
• Run integration tests against the latest Paperless version

[cocogitto-bot]

✔️ ea59b38...d6a90cc - Conventional commits check succeeded.

[codecov]

Codecov Report

Attention: 18 lines in your changes are missing coverage. Please review.

Comparison is base (f7c2319) 83.78% compared to head (d6a90cc) 90.49%.

Files	Patch %	Lines
...lksnis.PaperlessDotNet/Documents/DocumentClient.cs	71.42%	7 Missing and 5 partials ⚠️
...erlessDotNet/Serialization/HttpClientExtensions.cs	40.00%	2 Missing and 1 partial ⚠️
...aperlessDotNet/Documents/DocumentCreationResult.cs	50.00%	2 Missing ⚠️
...e/VMelnalksnis.PaperlessDotNet/Tasks/TaskClient.cs	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #151      +/-   ##
==========================================
+ Coverage   83.78%   90.49%   +6.71%     
==========================================
  Files          12       17       +5     
  Lines         148      242      +94     
  Branches        9       24      +15     
==========================================
+ Hits          124      219      +95     
+ Misses         19       13       -6     
- Partials        5       10       +5     

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