BigQuery code generator

[jskeet]

The code generator code itself is reasonably ugly, but that's not too much of a concern.
The second commit shows an example with four methods for label modification.
If we like this tool in general, I'll probably go back and apply it to the subset of existing methods that an easily be templated.

[codecov]

Codecov Report

Merging #1342 into master will decrease coverage by 0.13%.
The diff coverage is 26.66%.

@@            Coverage Diff             @@
##           master    #1342      +/-   ##
==========================================
- Coverage   61.79%   61.66%   -0.14%     
==========================================
  Files         292      294       +2     
  Lines        7882     7912      +30     
==========================================
+ Hits         4871     4879       +8     
- Misses       3011     3033      +22

Impacted Files	Coverage Δ
...V2/Google.Cloud.BigQuery.V2/ModifyLabelsOptions.cs	0% <0%> (ø)
....Cloud.BigQuery.V2/BigQueryClient.DatasetLabels.cs	33.33% <33.33%> (ø)

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[jskeet]

(If this looks good, I'll only merge the first commit, and put the rest in another PR that has the implementation as well.)

[jskeet]

Overnight I've worked out some ways I can make the code much cleaner. Should do exactly the same thing though... feel free to hold off reviewing until I've pinged this thread again.

[jskeet]

Okay, refactoring complete. Now most of the code works in terms of Parameter objects, which allows a lot more uniformity.

[jskeet]

The solution file changes are due to dotnet/project-system#1821

[chrisdunelm]

Crude, but better than copy/pasting :)
LGTM

[jskeet]

(Reduced to one commit, so no actual generated code. Will merge on green, then start working on the labels feature...)