Tweaks to custom metadata provider schema

[ahobsonsayers]

When developing my custom metadata provider, I noticed a couple of issues with the provider schema that I thought I would fix. I also did a bit of minor tidy up (I hope this is ok, If not let me know and I will revert!)

Schema changes:

• duration type should be an integer not a number
• I have split out SeriesMetadata to its own component, as this will generate its own type for those who use generators with the schema, instead of nested anonymous objects.
• series under SeriesMetadata being required was specified in the incorrect way using required: <bool> instead of required: <property array>
• sequence under SeriesMetadata should not be a number, it should be a string to allow for sequences like 0.5, 1-3 and 1,3,5

Let me know if there is any changes you would like me to make!

[advplyr]

LGTM thanks!