Java client, simplifies ApiConfiguration servers info + adds operation code sample

[spacether]

Java client

• serverIndexInfo added added in ApiConfiguration
• stops using EnumMap in ServerInfo and SecurityInfo classes to fix checker framework warning
• separate documentation files added for server variables schema input
• adds operation code sample
• updates all exceptions to inherit from Exception, so they must be thrown and handled
• where exceptions should never happen, exceptions are changed to RuntimeException: for example getting required properties out of map output classes, the values will always be the correct type. But the map data store does not know this. Another example, configuration securityIndex/serverIndex/securityRequiremntObject fetching. If these subclassed Exception, then all operation invocations would need to explicitly handle the case where configuration inputs were not set up correctly.

Future

• add docs for api configuration
• add apiClient for each path
• add apiClient that uses operationId

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.
• Run the following to build the project and update samples:

  mvn clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/generate_samples_configs/python*
  For Windows users, please run the script in Git BASH.