[Java/okhttp] Add dynamic operations option

[cbornet]

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.
• If contributing template-only or documentation-only changes which will change sample output, build the project beforehand.
• Run the shell script ./bin/generate-samples.shto update all Petstore samples related to your fix. 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/configs/java*. For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

This PR adds support for dynamic operations.
When the option is selected, the generated code will get the details for the operations from an OAS that is provided at runtime. The generated code will look for operationId and the parameter names in this OAS to build the requests.
This way the client is more decoupled from the server. You can make changes in the server API and the client will still work without needing to be regenerated. This is a major gain when ensuring the client is up-to-date is hard (eg. mobile apps).

Examples of API changes that keep compatibility with dynamic clients (and that break non-dynamic clients):

• Changing the operation URL (eg changing /api/user/{id} to /api/users/{id})
• Changing the operation method (eg. a POST becomes a PUT)
• Moving an operation parameter between path, header and cookie

@bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10) @bkabrda (2020/01)

[wing328]

Given that the feature is off by default, I'm happy to include it in the upcoming 5.0.0 release.