feat(model): dataset model support

[callmehiphop]

This adds Dataset Model support.

[callmehiphop]

@tswast @shollyman I think I have the basic implementation of the CRUD methods complete, but I'm not sure how to test it? I tried reviewing googleapis/google-cloud-python#7562 to see how models are created there, but my python isn't so great that I could figure it out 😢. Any hints here would be greatly appreciated! Found a go example!

@sduskis @JustinBeckwith Should I include samples in this PR? Historically some one else has always written them, but it looks like I can pull the region tags from the PRs in other langs.

[tswast]

Should I include samples in this PR? Historically some one else has always written them, but it looks like I can pull the region tags from the PRs in other langs.

Either you or @steffnay should write the samples. I think Steffany wanted to focus on Node.js samples this quarter.

[codecov]

Codecov Report

Merging #449 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #449      +/-   ##
==========================================
+ Coverage    99.4%   99.42%   +0.02%     
==========================================
  Files           4        5       +1     
  Lines         674      699      +25     
  Branches      189      194       +5     
==========================================
+ Hits          670      695      +25     
  Misses          2        2              
  Partials        2        2

Impacted Files	Coverage Δ
src/index.ts	98.49% <100%> (+0.01%)	⬆️
src/dataset.ts	100% <100%> (ø)	⬆️
src/model.ts	100% <100%> (ø)

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[steffnay]

This is great. I can do the samples!