[JAVA][FEIGN] Implement unit tests for java-feign client

[hugo-ma-alves]

Hello

This PR includes the following changes

• Unit tests for java feign client
• Implement the unit tests with Junit5
• Set default feign http client to OkHttp, the default feign client does not support Patch verb

cc:

@bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10) @nmuesch (2021/01)

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[wing328]

CircleCI failure not related to this change.

[wing328]

Notice that the indentation has changed from 4-space to 2-space in the test files. Users are advised to use a code formatter (e.g. prettier.io) to automatically format the auto-generated code to make the code conforms to code style set by the users.