NIFI-8497: implemented SlackRecordSink and Test, wrote documentation …

[emiliosetiadarma]

…in CapabilityDescription of SlackRecordSink

Summary

NIFI-8497

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[exceptionfactory]

Thanks for the contribution @emiliosetiadarma, this looks like a useful addition.

Although the new SlackRecordSink follows a similar pattern as the existing Slack components, this is an opportunity to move away from the javax.json and Apache HttpClient libraries. Instead, the Jackson JSON library should be used, along with the WebServiceClientProvider and nifi-web-client-api, which provides an abstraction around HTTP client operations. Eventually the other Slack components should be refactored along the same lines, but that can be handled separately. As this is a new component, it should make use of these features.

[exceptionfactory]

Thanks for making the updates @emiliosetiadarma, I noted a few more recommendations, but this looks close to completion.

[exceptionfactory]

Thanks for making the updates @emiliosetiadarma, I pushed one more commit to remove the Message and Attachment model classes, since they were not used in any of the service classes.

[exceptionfactory]

Merging following successful builds. Thanks for the work on this feature @emiliosetiadarma!