Tools: adds actions to create client code

[llbrt]

• add actions to create API client source code
  locally when the Google API client code
  generator is installed (command generate_library)

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This change is 

[codecov]

Codecov Report

Merging #52 into master will decrease coverage by 0.03%.
The diff coverage is 79.41%.

@@             Coverage Diff              @@
##             master      #52      +/-   ##
============================================
- Coverage     82.51%   82.47%   -0.04%     
- Complexity     1827     1836       +9     
============================================
  Files           156      159       +3     
  Lines          6621     6689      +68     
  Branches        878      881       +3     
============================================
+ Hits           5463     5517      +54     
- Misses          846      859      +13     
- Partials        312      313       +1

Impacted Files	Coverage Δ	Complexity Δ
...com/google/api/server/spi/tools/EndpointsTool.java	88.37% <100%> (+0.56%)	8 <0> (ø)	⬇️
...oogle/api/server/spi/tools/GenClientSrcAction.java	60% <60%> (ø)	2 <2> (?)
...oogle/api/server/spi/tools/GetClientSrcAction.java	78.57% <78.57%> (ø)	3 <3> (?)
.../api/server/spi/tools/LocalClientLibGenerator.java	91.3% <91.3%> (ø)	4 <4> (?)

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[clementdenis]

Reviewed 5 of 5 files at r1, 3 of 3 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved