Add GraphQL mutation to create release asset link

What does this MR do?

Adds a GraphQL mutation - releaseAssetLinkCreate - to create a new release asset link.

This new mutation is equivalent to the existing REST API creation endpoint:

POST /projects/:id/releases/:tag_name/assets/links

Example query/response

When a link is successfully created

Query:

mutation {
  releaseAssetLinkCreate(input: {
    projectPath: "root/release-test", 
    tagName: "v4.5", 
    name: "a new asset link", 
    url: "https://example.com/a-new-asset-link"
  }) {
    link {
      id
    }
    errors
  }
}

Response:

{
  "data": {
    "releaseAssetLinkCreate": {
      "link": {
        "id": "gid://gitlab/Releases::Link/184"
      },
      "errors": []
    }
  }
}

When the project doesn't exist

Query:

mutation {
  releaseAssetLinkCreate(input: {
    projectPath: "root/fake-project",
    tagName: "fake-tag",
    name: "my new link",
    url: "https://example.com/new-link"
  }) {
    link {
      id
    }
    errors
  }
}

Response:

{
  "data": {
    "releaseAssetLinkCreate": null
  },
  "errors": [
    {
      "message": "The resource that you are attempting to access does not exist or you don't have permission to perform this action",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "path": [
        "releaseAssetLinkCreate"
      ]
    }
  ]
}

When the release doesn't exist

Query:

mutation {
  releaseAssetLinkCreate(input: {
    projectPath: "root/release-test",
    tagName: "fake-tag",
    name: "my new link",
    url: "https://example.com/new-link"
  }) {
    link {
      id
    }
    errors
  }
}

Response:

{
  "data": {
    "releaseAssetLinkCreate": {
      "link": null,
      "errors": [
        "Release with tag \"fake-tag\" was not found"
      ]
    }
  }
}

When a validation error occurs

Query:

mutation {
  releaseAssetLinkCreate(input: {
    projectPath: "root/release-test",
    tagName: "v4.5",
    name: "my new link",
    url: "https://example.com/new-link"
  }) {
    link {
      id
    }
    errors
  }
}

Response:

 {
  "data": {
    "releaseAssetLinkCreate": {
      "link": null,
      "errors": [
        "Url has already been taken",
        "Name has already been taken"
      ]
    }
  }
}

Permissions

Permissions for this endpoint mirror the existing REST API endpoint.