[Java/Jersey2] Use Map interface for configureApiKeys

[nmuesch]

Use Map instead of HashMap for the configureApiKeys method in the Java client. This allows users to pass in whatever implementation of Map they'd like. This pattern is used elsewhere throughout the java client currently.

This addresses an issue raised here - DataDog/datadog-api-client-java#513

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.
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[nmuesch]

Copying the technical committee for Java:
@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) @bkabrda (2020/01)

[nmuesch]

Hey, just wanted to followup here to see if there was anything I could do to further this. Thanks!

[bkabrda]

LGTM!