Support Types key inside the json parser

[JGiola]

Is your feature request related to a problem? Please describe.

A common configuration shared by all the parsers is the Types configuration that instruct it on the types of certain fields. This was originally intended to use in conjunction with the regex parser to cast fields from the string type.

This knowledge is totally missing in the documentation, and the user is led to believe that can be used on every parser to force types on the parsed data even with the json parser.
This is not true if you search in the code for the flb_parser_typecast function is called inside all the concrete implementation of the parser except in src/flb_parser_json.c.

Describe the solution you'd like
A nice solution is to add the same typecast function in the json parser code to share the same function across all the parsers.
This can be useful for cast types from different json log to common types (the more common, cast to string a timestamp saved as a number to pass it to strptime)

Describe alternatives you've considered
Clearly states in the documentation that this is working as intended and it can be used only with the other parser (like the Decode_Fieldkey that on the contrary work only on json parsers).

Additional context
I was trying to cast some values inside a json log but they where never picked up. I've gained some frustration in exchange of time to find why.

[nokute78]

Types is an option for the formats which don't support data types. e.g. ltsv.
On the other hand, JSON can represent data types. (integers, floats, strings, booleans.)
So, JSON parser casts them automatically and it doesn't support types option.

[nokute78]

@JGiola

Clearly states in the documentation that this is working as intended and it can be used only with the other parser (like the Decode_Fieldkey that on the contrary work only on json parsers).

Sorry for undocumented it.
I sent a patch for document to make clear which parser supports Types option.

fluent/fluent-bit-docs#211