Create AssetMarshaller for asset link parsing

As part of #66 (closed), we're going to add the possibility to add arrays of asset links, in addition to passing a single asset link.

The new AssetMarshaller function introduced by this MR here will help to parse whether an array assets or a single asset (or both) are passed, upon running release-cli create.

Manual testing

Create a release with one asset

Create a release with three assets in an array

Create a release with two assets in an array and one single asset on its own

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