Rename DataView Metadata to Annotations.

[eerhardt]

I have ensured there is no public API with the word metadata in it anymore. The only place this occurs is in the Microsoft.ML.UniversalModelFormat.Onnx namespace, which is a different type of "metadata".

There are still internal/private usages of the term "metadata", but this PR is large enough. We can fix those in a subsequent change.

Fix #1843
Fix #2297

[codecov]

Codecov Report

Merging #2712 into master will decrease coverage by <.01%.
The diff coverage is 80.98%.

@@            Coverage Diff             @@
##           master    #2712      +/-   ##
==========================================
- Coverage   71.66%   71.66%   -0.01%     
==========================================
  Files         808      808              
  Lines      142253   142254       +1     
  Branches    16138    16138              
==========================================
- Hits       101950   101948       -2     
- Misses      35864    35866       +2     
- Partials     4439     4440       +1

Flag	Coverage Δ
#Debug	71.66% <80.98%> (-0.01%)	⬇️
#production	67.91% <77.15%> (-0.01%)	⬇️
#test	85.85% <94.92%> (ø)	⬆️

Impacted Files	Coverage Δ
src/Microsoft.ML.Core/Data/IHostEnvironment.cs	95.12% <ø> (ø)	⬆️
src/Microsoft.ML.Data/Transforms/TransformBase.cs	56.07% <ø> (ø)	⬆️
.../Microsoft.ML.StaticPipe/SchemaAssertionContext.cs	45.65% <ø> (ø)	⬆️
...rosoft.ML.Data/Scorers/QuantileRegressionScorer.cs	100% <ø> (ø)	⬆️
...Microsoft.ML.Transforms/OptionalColumnTransform.cs	28.16% <ø> (ø)	⬆️
...rc/Microsoft.ML.Core/EntryPoints/PredictorModel.cs	100% <ø> (ø)	⬆️
src/Microsoft.ML.StaticPipe/PipelineColumn.cs	71.05% <ø> (ø)	⬆️
....Data/Evaluators/MultiOutputRegressionEvaluator.cs	0.37% <0%> (ø)	⬆️
...ata/Transforms/ValueToKeyMappingTransformerImpl.cs	81.38% <0%> (ø)	⬆️
src/Microsoft.ML.LightGBM/LightGbmTrainerBase.cs	62% <0%> (ø)	⬆️
... and 143 more

[Ivanidzo4ka]

[TomFinley]

This is quite the PR @eerhardt. Thank you for not only renaming the class, but making sure to also handle so much of the documentation (even internal documentation), parameter names, etc.