Adding JsonPath Support

[hattesen]

Copied from Plan for supporting JsonPath #1997

The idea of ArduinoJson providing support for a query language like JsonPath has been discussed several times in the past¹²³, and sample code fragments/examples⁴ have been published.

While full JsonPath support would certainly be "nice", a small subset would be a great starting point.

Any such query feature should not add bloat to the core code, but would be well suited to be implemented as part of a (JsonPath or similar) utility class.

A JsonPath branch should be created from [ArduinoJson/7.x](https://github.com/bblanchon/ArduinoJson/tree/7.x), to allow experimentation with JsonPath-related API design and implementation, providing a simple API allowing (non-compiled) to query a JsonDocument (or JsonVariant) instance.

Rather than inventing a new/custom syntax, the query implementation should support a well-defined subset of JsonPath subset, returning a JsonVariant, or JsonArray of matching elements. An optional filter argument (like in deserializeJson() Filtering), would probably be useful to include as well

The initial work of this JsonPath support feature should focus on:

1. Specifying a "query"/JsonPath support API, allowing flexibility for future feature extension (e.g. compiled JsonPath queries)
2. Propose a prioritized list of JsonPath features and/or API support
3. Propose a "minimum viable" JsonPath subset and API to target for an initial implementation

I would be happy to contribute to any specification/implementation effort, but would like the above points to be settled before "breaking ground" on any implementation.

Let me hear your views!

Footnotes

1. JSONPath bblanchon/ArduinoJson#821 ↩

2. Function to resolve a JSON path composed of several keys bblanchon/ArduinoJson#1505 ↩

3. Concatenated Names bblanchon/ArduinoJson#1904 ↩

4. Concatenated Names bblanchon/ArduinoJson#1904 element.element support fragment ↩

[hattesen]

See JsonPath Support for documentation of the propposed JsonPath support