Add Config DSL

[blowmage]

This PR is an alternate implementation of the Configuration library. It provides a single module that clients can use to easily create a Configuration class. It is parent aware, so nested/child configuration objects can be created that will look to the parent object for values when local values are not present.

[codecov[bot]]

Codecov Report

Merging #196 into master will decrease coverage by 69.36%. The diff coverage is 28.4%.

@@             Coverage Diff             @@
##           master     #196       +/-   ##
===========================================
- Coverage    97.7%   28.34%   -69.37%     
===========================================
  Files          38       44        +6     
  Lines        2139     2565      +426     
===========================================
- Hits         2090      727     -1363     
- Misses         49     1838     +1789

Impacted Files	Coverage Δ
test/google/gax/config/errors_test.rb	100% <100%> (ø)
test/google/gax/config/attrs_test.rb	18.43% <18.43%> (ø)
test/google/gax/config/method_test.rb	18.47% <18.47%> (ø)
test/google/gax/config/nested_test.rb	19.83% <19.83%> (ø)
lib/google/gax/config.rb	46.66% <46.66%> (ø)
lib/google/gax/config/method.rb	81.81% <81.81%> (ø)
test/google/gax/api_call/retry/retry_test.rb	8.19% <0%> (-90.17%)	:arrow_down:
test/google/gax/api_call/retry/raise_test.rb	11.76% <0%> (-88.24%)	:arrow_down:
test/google/gax/configuration/derive_test.rb	9.32% <0%> (-85.6%)	:arrow_down:
test/google/gax/api_call/raise_test.rb	15.38% <0%> (-84.62%)	:arrow_down:
... and 37 more

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