feat: add new fields in asset model

[scortier]

Added new fields on Asset to accomodate the new model from Meteor

1. url: string

2. attributes: map[string]interface{}

[coveralls]

Pull Request Test Coverage Report for Build 2758656324

• 36 of 43 (83.72%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 79.643%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core/asset/patch.go	11	18	61.11%
<!--	Total:	36	43

Totals
Change from base Build 2654050687:	-0.02%
Covered Lines:	4511
Relevant Lines:	5664

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💛 - Coveralls