Add json path operator

[kairichard]

Why add this?

The default decode JSON operator requires the user to build a matching type that exactly mimics the schema in the JSON document. However, if the user only wants to extract a single value or a small subset she still has to construct the entire type.

The new operator JSON Path allows the user to specify multiple JSON path queries to extract data. It uses https://github.com/tidwall/gjson for parsing and extracting the data.

There is a noteworthy case where this operator might not work as expected.
When the path expression would return a value that is not a primitive but an object or an array it actually returns the raw JSON for the given expression. Reasons for implementing it that way were:

• to handle such a case the user would again need to specify which of the expression are expected to return an object or an array, which would defeat the goal of this change.
• when returning a string we can use the old decode JSON to work with a much smaller document

//cc @td5r
related to #237

[codecov-io]

Codecov Report

Merging #238 into master will increase coverage by 0.21%.
The diff coverage is 88%.

@@            Coverage Diff             @@
##           master     #238      +/-   ##
==========================================
+ Coverage    41.7%   41.92%   +0.21%     
==========================================
  Files         102      103       +1     
  Lines        5361     5386      +25     
==========================================
+ Hits         2236     2258      +22     
- Misses       2946     2948       +2     
- Partials      179      180       +1

Impacted Files	Coverage Δ
pkg/elem/manager.go	95% <100%> (+0.04%)	⬆️
pkg/elem/encoding_json_path.go	87.5% <87.5%> (ø)

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