feat(core): add TensorBoard log dir watcher

[kptkin]

Description

• Fixes WB-NNNNN
• Fixes #NNNN

Implement basic Tensorboard sync feature in wandb-core: tfevent files will now be synced to the run so that a TB tab will be available in the FE.

• Generalize the watcher to support custom handle functions
• Use it to implement a TB log dir watcher that tracks the TB event files so that we upload them at the end of a run.

Testing

How was this PR tested?

[codecov]

Codecov Report

Attention: 103 lines in your changes are missing coverage. Please review.

Comparison is base (f513753) 79.46% compared to head (67a7fef) 79.33%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6769      +/-   ##
==========================================
- Coverage   79.46%   79.33%   -0.13%     
==========================================
  Files         447      450       +3     
  Lines       49667    49798     +131     
==========================================
+ Hits        39466    39509      +43     
- Misses       9955    10041      +86     
- Partials      246      248       +2     

Flag	Coverage Δ
func	52.13% <ø> (-0.01%)	⬇️
system	63.89% <50.95%> (-0.11%)	⬇️
unit	58.72% <28.57%> (-0.07%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
core/internal/watcher/registry.go	100.00% <100.00%> (ø)
core/pkg/monitor/monitor.go	83.56% <ø> (ø)
core/pkg/server/files.go	85.03% <100.00%> (+1.95%)	⬆️
core/pkg/server/stream.go	100.00% <100.00%> (ø)
core/internal/watcher/event.go	50.00% <50.00%> (ø)
core/pkg/server/handler.go	78.78% <45.45%> (-0.57%)	⬇️
core/internal/watcher/watcher.go	65.62% <63.41%> (-12.79%)	⬇️
core/pkg/server/tb.go	18.18% <18.18%> (ø)

... and 17 files with indirect coverage changes

[dmitryduev]

🎁🎄🎁