Add model name mappings support to the Elixir client generator

[wing328]

all tests passed:

==> mime
Compiling 1 file (.ex)
Generated mime app
==> jason
Compiling 10 files (.ex)
Generated jason app
==> tesla
Compiling 34 files (.ex)
Generated tesla app
==> erlex
Compiling 2 files (.erl)
Compiling 1 file (.ex)
Generated erlex app
==> dialyxir
Compiling 64 files (.ex)
Generated dialyxir app
==> openapi_petstore
Compiling 57 files (.ex)
Generated openapi_petstore app
..........
Finished in 0.2 seconds (0.2s async, 0.00s sync)
10 tests, 0 failures

Randomized with seed 81276

Similar PR for C# #16209

cc @mrmstn (2018/12)

### PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
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  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
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• In case you are adding a new generator, run the following additional script :

  ./bin/utils/ensure-up-to-date
  

  Commit all changed files.
• File the PR against the correct branch: master (6.3.0) (minor release - breaking changes with fallbacks), 7.0.x (breaking changes without fallbacks)
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[mrmstn]

LGTM, Thank you @wing328