Add generated code for Text to Speech service

[lpatino10]

This PR adds the generated models and service class for Speech to Text.

All unit and integration tests have been updated to reflect the changes, and new ones have been added as necessary as well.

This service shouldn't require any manual changes post-generation.

[codecov-io]

Codecov Report

Merging #858 into develop will decrease coverage by 0.24%.
The diff coverage is 65.18%.

@@              Coverage Diff              @@
##             develop     #858      +/-   ##
=============================================
- Coverage      56.23%   55.98%   -0.25%     
- Complexity      2214     2255      +41     
=============================================
  Files            496      514      +18     
  Lines          11993    12317     +324     
  Branches         675      684       +9     
=============================================
+ Hits            6744     6896     +152     
- Misses          4856     5024     +168     
- Partials         393      397       +4

Impacted Files	Coverage Δ	Complexity Δ
...r_cloud/text_to_speech/v1/model/Pronunciation.java	50% <ø> (ø)	2 <0> (ø)	⬇️
...oud/text_to_speech/v1/model/ListVoicesOptions.java	0% <0%> (ø)	0 <0> (?)
...oper_cloud/text_to_speech/v1/model/CustomWord.java	0% <0%> (ø)	0 <0> (?)
...per_cloud/text_to_speech/v1/model/VoiceModels.java	100% <100%> (ø)	3 <3> (?)
.../developer_cloud/text_to_speech/v1/model/Word.java	100% <100%> (ø)	7 <7> (?)
...oud/text_to_speech/v1/model/SupportedFeatures.java	14.28% <14.28%> (ø)	1 <1> (?)
...cloud/text_to_speech/v1/model/AddWordsOptions.java	48.27% <48.27%> (ø)	3 <3> (?)
...per_cloud/text_to_speech/v1/model/Translation.java	50% <50%> (ø)	3 <3> (?)
...developer_cloud/text_to_speech/v1/model/Words.java	50% <50%> (ø)	2 <2> (?)
..._cloud/text_to_speech/v1/model/AddWordOptions.java	56.75% <56.75%> (ø)	5 <5> (?)
... and 66 more

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[germanattanasio]

remove the javadoc starting from API Overview