Exporter: Ignore workspace assets of deleted users and service principals

[alexott]

Changes

Don't export workspace objects (notebooks, workspace files, and directories) of deleted users and service principals. Otherwise, when importing into another workspace it will be attempted to create them, and the apply step will fail as we can't create folders under the /Users folder.

It doesn't cover other objects of deleted users, like SQL queries/alerts/dashboards - this may come in the next PRs.

Tests

• make test run locally
• relevant change in docs/ folder
• covered with integration tests in internal/acceptance
• relevant acceptance tests are passing
• using Go SDK

[codecov-commenter]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (0d6281c) 84.11% compared to head (7176010) 84.08%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2980      +/-   ##
==========================================
- Coverage   84.11%   84.08%   -0.03%     
==========================================
  Files         159      159              
  Lines       13929    13937       +8     
==========================================
+ Hits        11716    11719       +3     
  Misses       1546     1546              
- Partials      667      672       +5     

Files	Coverage Δ
exporter/command.go	82.60% <100.00%> (+0.38%)	⬆️
exporter/context.go	79.38% <ø> (ø)
exporter/importables.go	73.93% <66.66%> (-0.14%)	⬇️
exporter/util.go	77.66% <87.80%> (-0.43%)	⬇️