add jsonschema_is_valid function and its tests

[imor]

What kind of change does this PR introduce?

Feature

What is the current behavior?

Fixes #28

What is the new behavior?

This PR adds a function jsonschema_is_valid(schema: Json) which returns true/false if the schema is valid/invalid. If the schema is invalid it also raises a notice which tells the user which part of the schema is invalid. For example when a valid schema is provided it return true:

pg_jsonschema=# select jsonschema_is_valid('{"type": "object", "properties": { "tags": { "type": "array", "items": { "type": "string", "maxLength": 16 } } } }');
 jsonschema_is_valid
---------------------
 t
(1 row)

And when an invalid schema is provided (notice typo sting instead of string in input schema):

pg_jsonschema=# select jsonschema_is_valid('{"type": "object", "properties": { "tags": { "type": "array", "items": { "type": "sting", "maxLength": 16 } } } }');
NOTICE:  Invalid JSON schema at path: /properties/tags/items
 jsonschema_is_valid
---------------------
 f
(1 row)

Additional context

[olirice]

thanks, this looks great. I'll check it out tomorrow and follow up then

[olirice]

could you also add this function to README.md
in the same style as the existing functions?

[olirice]

instance_path is not always available

pg_jsonschema=# select jsonschema_is_valid('"sdfg"');   
NOTICE:  Invalid JSON schema at path:               

could you skip the notice when no path is available?

[imor]

Done.

[imor]

could you also add this function to README.md in the same style as the existing functions?

Added to the README.

[olirice]

thanks!