feat(v1beta2): generate v1beta2 client

[chingor13]

Also renames to google-cloud-document-ai (not released yet so not a breaking change)

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@6a2e2ff). Click here to learn what that means.
The diff coverage is 67.69%.

@@            Coverage Diff            @@
##             master       #9   +/-   ##
=========================================
  Coverage          ?   67.69%           
  Complexity        ?       36           
=========================================
  Files             ?        6           
  Lines             ?      260           
  Branches          ?        1           
=========================================
  Hits              ?      176           
  Misses            ?       83           
  Partials          ?        1

Impacted Files	Coverage Δ	Complexity Δ
.../v1beta2/DocumentUnderstandingServiceSettings.java	20% <20%> (ø)	2 <2> (?)
...v1beta2/stub/DocumentUnderstandingServiceStub.java	20% <20%> (ø)	1 <1> (?)
...ai/v1beta2/DocumentUnderstandingServiceClient.java	45.45% <45.45%> (ø)	8 <8> (?)
...pcDocumentUnderstandingServiceCallableFactory.java	50% <50%> (ø)	4 <4> (?)
...stub/DocumentUnderstandingServiceStubSettings.java	82.45% <82.45%> (ø)	12 <12> (?)
...ta2/stub/GrpcDocumentUnderstandingServiceStub.java	84.61% <84.61%> (ø)	9 <9> (?)

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