mcp: add ResourceLink type (#76)

This PR introduces the ResourceLink type to represent links to
resources. The new type includes the following fields: `uri`, `name`,
`title`, `description`, `mimeType`, `size`, and `_meta`.

This implementation conforms to the `ResourceLink` schema introduced in
the [MCP schema
2025-06-18](1cfdf1e7a8/schema/2025-06-18/schema.json (L1826)).

Note: `ResourceLink` extends from the `Resource` type in the MCP schema.
As such, it inherits support for optional fields like `title`, `_meta`,
etc. Thus, `URI`, `Name`, `Title`, `Description`, and `Size` are added
to the `wireContent`

For reference, the [TypeScript SDK test
suite](fb4d07f0ee/src/types.test.ts (L43))
provides examples demonstrating valid ResourceLink objects with all
optional fields populated.

https://modelcontextprotocol.io/specification/2025-06-18/server/tools#resource-links
- [ResourceLink JSON
Schema](1cfdf1e7a8/schema/2025-06-18/schema.json (L1826))

Signed-off-by: Xin Fu <xfu83@bloomberg.net>

Fixes #14.

mcp/content_test.go

@@ -82,6 +82,24 @@ func TestContent(t *testing.T) {
 			},
 			`{"type":"resource","resource":{"uri":"file://foo","mimeType":"text","text":"abc"},"_meta":{"key":"value"},"annotations":{"priority":1}}`,
 		},
+		{
+			&mcp.ResourceLink{
+				URI:  "file:///path/to/file.txt",
+				Name: "file.txt",
+			},
+			`{"type":"resource_link","uri":"file:///path/to/file.txt","name":"file.txt"}`,
+		},
+		{
+			&mcp.ResourceLink{
+				URI:         "https://example.com/resource",
+				Name:        "Example Resource",
+				Title:       "A comprehensive example resource",
+				Description: "This resource demonstrates all fields",
+				MIMEType:    "text/plain",
+				Meta:        mcp.Meta{"custom": "metadata"},
+			},
+			`{"type":"resource_link","mimeType":"text/plain","uri":"https://example.com/resource","name":"Example Resource","title":"A comprehensive example resource","description":"This resource demonstrates all fields","_meta":{"custom":"metadata"}}`,
+		},
 	}
 
 	for _, test := range tests {