first() and all(): Add syntactic sugar to treat ServerStreamingCallables like UnaryCallables

[igorbernstein2]

This PR adds first() and all() callables on top of ServerStreamingCallables. This allows for simple interaction with short server streams. Example usage:

Row row = client.readRowsCallable().first().call(request)
ApiFuture<List<Row>> rows = client.readRowsCallable().all().futureCall(request)

[codecov-io]

Codecov Report

Merging #444 into master will increase coverage by 0.45%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #444      +/-   ##
============================================
+ Coverage     71.48%   71.94%   +0.45%     
- Complexity      728      746      +18     
============================================
  Files           150      154       +4     
  Lines          3276     3329      +53     
  Branches        256      256              
============================================
+ Hits           2342     2395      +53     
  Misses          837      837              
  Partials         97       97

Impacted Files	Coverage Δ	Complexity Δ
...om/google/api/gax/rpc/ServerStreamingCallable.java	50% <100%> (+9.25%)	7 <3> (+2)	⬆️
...ogle/api/gax/rpc/FirstElementResponseObserver.java	100% <100%> (ø)	6 <6> (?)
...a/com/google/api/gax/rpc/FirstElementCallable.java	100% <100%> (ø)	2 <2> (?)
.../java/com/google/api/gax/rpc/SpoolingCallable.java	100% <100%> (ø)	2 <2> (?)
...m/google/api/gax/rpc/SpoolingResponseObserver.java	100% <100%> (ø)	6 <6> (?)

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[igorbernstein2]

rebased

[garrettjonesgoogle]

LGTM