feat: accept $schema property

[pierrecamilleri]

Adds $schema reading from descriptor and writing to_descriptor for datapackage v2 :

• package.$schema
• resource.$schema
• schema.$schema
• dialect.$schema

A root level [xxx] descriptor MAY have a $schema property

No validation is performed against the $schema properties yet.

implementation details

Implemented on Metadata (_schema_profile property, to not clash with e.g. resource.schema and as $schema is not a valid python symbol), but only child classes with explicit declaration will read it (for others, it will be considered a custom property).

[zaneselvans]

Thanks for adding this! I recently found that I had to manually add this to some new v2 datapackages I was writing out and was very surprised.