Metadata helpers

[stephenwf]

Similar to refs (#8) metadata is a field type that could be cumbersome to update all at once (in a reactive setting).
Where you currently can only replace the entire metadata value:

vault.setEntityValue(manifest, 'metadata', newArray);

We could split this:

vault.addMetadata(manifest, atIndex);
vault.removeMetadata(manifest, index);
vault.updateMetadata(manifest, atIndex, newValue);
vault.reorderMetadata(manifest, startIndex, endIndex);