Unmarshal Media interface

[vinhluan]

Since gqlgen generates Go interfaces for GraphQL interfaces, leading to an error when unmarshalling JSON data contains interface types. This pull request introduces custom UnmarshalJSON methods to unmarshal Media interface.

Related issue: 99designs/gqlgen#2446

Example query with this issue:

query products {
	products(first: 10) {
		id
		title
		handle
		media {
			edges {
				node {
					mediaContentType
					...on MediaImage {
						id
						alt
						mimeType
						image {
							height
							src
							width
						}
					}
					...on Model3d {
						id
						alt
						originalSource {
							url
							format
							filesize
							mimeType
						}
						preview {
							image {
								src
							}
						}
					}
					...on Video {
						id
						alt
						duration
						originalSource {
							url
							format
							mimeType
 							height
							width
						}
						preview {
							image {
								src
							}
						}
					}
					...on ExternalVideo {
						id
						originUrl
						embedUrl
						preview {
							image {
								src
							}
						}
					}
				}
			}
		}
	}
}