[ANN] Better JSON parsing

[Arran Cudbard-Bell]

v3.1.x rlm_json module registers a map function to allow mapping fields from
JSON structures to attributes.

The path through the JSON object is specified with fr jpath, which is
the FreeRADIUS implementation of the jpath grammar described here:

	http://goessner.net/articles/JsonPath/

rlm_json currently implements a subset of the grammar described by the
above implementation.

Selectors currently implemented are:
  - $          Root node (only valid at the start of the path).
  - @          Current node (only valid at the start of the path).
  - .<name>    A field within an object.
  - [<idx>]    Index within an array.
  - [<start>:<end>[:<step>]]  A slice within an array (identical to the Python syntax).
  - [<idx>,<start>:<end>]     Multiple indexes/slices within an array.
  - .*         All the children of the current node

Automatic casting will occur between JSON and attribute types where possible.

FreeRADIUS does not currently have a signed64 or floating point type, to map
large signed numbers and JSON doubles to.  These may instead be written to
string type attributes.

Assignment of JSON objects/arrays to strings is supported, in which case the
JSON serialized form of the object/array is used.

If a jpath matches multiple nodes, unless the map includes the += operator
only the first node's value will be used.
If the map uses += then multiple instances of the attribute will be created,
each holding a different node value.

Simple example:
{
	"user": "bob",
	"account number": 7124503,
	"groups": {
		"admin",
		"networks",
		"bob"
	}

}

map json "%{rest:GET http://example.org/api/user/%{User-Name}" {
	&Tmp-Integer-0 	:= '$.account number'
	&Group		+= '$.groups.*"
}
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