Add support to httpjson input plugin to parse nested objects and arrays of objects

[kevinjos]

Required for all PRs:

• CHANGELOG.md updated (we recommend not updating this until the PR has been approved by a maintainer)
• Sign CLA (if not already signed)
• README.md updated (if adding a new plugin)

[sparrc]

thank you for the PR @kevinjos, but this is not quite generic enough. The purpose of JSON paths is to be able to parse any JSON into influx line-protocol. Your change only deals with specifying a path to a nested array, which is exactly the type of one-off changes that I'm trying to avoid.

[kevinjos]

Thank you for the feedback @sparrc

I see the "flatten everything up to a float" approach as very generic. The weakness is that it does not take advantage of tags. What are other weaknesses in the current parser that a more generic parser should solve? Also, what are additional use-cases for JSON paths in the JSON parser besides allowing the user to configure paths to metric objects?