Move the builder classes in DataViewSchema

[eerhardt]

• Move MetadataBuilder to be DataViewSchema.Metadata.Builder.
• Move SchemaBuilder to DataViewSchema.Builder.
• Rename GetMetadata and GetSchema to ToMetadata and ToSchema

Working towards #2297

There are 2 tasks left before that issue can be closed.

1. Rename 'metadata' to 'annotations' - a very large change.
2. Remove anything from DataViewRowId that makes it seem like a number.

[rogancarr]

🚗 🎈

[codecov]

Codecov Report

Merging #2703 into master will decrease coverage by 0.01%.
The diff coverage is 89%.

@@            Coverage Diff             @@
##           master    #2703      +/-   ##
==========================================
- Coverage   71.59%   71.58%   -0.02%     
==========================================
  Files         805      805              
  Lines      142004   142004              
  Branches    16119    16119              
==========================================
- Hits       101673   101654      -19     
- Misses      35891    35914      +23     
+ Partials     4440     4436       -4

Flag	Coverage Δ
#Debug	71.58% <89%> (-0.02%)	⬇️
#production	67.87% <88.62%> (-0.02%)	⬇️
#test	85.73% <91.66%> (ø)	⬆️

Impacted Files	Coverage Δ
...ata/Transforms/ValueToKeyMappingTransformerImpl.cs	81.38% <ø> (ø)	⬆️
...icrosoft.ML.Core/Data/MetadataBuilderExtensions.cs	100% <ø> (ø)	⬆️
....Data/Evaluators/MultiOutputRegressionEvaluator.cs	0.37% <0%> (ø)	⬆️
test/Microsoft.ML.Benchmarks/HashBench.cs	0% <0%> (ø)	⬆️
...osoft.ML.Data/Depricated/Instances/HeaderSchema.cs	0% <0%> (ø)	⬆️
...icrosoft.ML.Data/Evaluators/ClusteringEvaluator.cs	53.22% <0%> (ø)	⬆️
...osoft.ML.Transforms/Text/TokenizingByCharacters.cs	95.32% <100%> (ø)	⬆️
src/Microsoft.ML.FastTree/FastTree.cs	80.76% <100%> (ø)	⬆️
....ML.Data/Scorers/FeatureContributionCalculation.cs	94.39% <100%> (ø)	⬆️
....ML.Data/Evaluators/QuantileRegressionEvaluator.cs	68.66% <100%> (ø)	⬆️
... and 44 more

[Ivanidzo4ka]