Allow `EmbeddedResource` and `ResourceLink` to be returned from an MCP tool

[Azquelt]

For each type:

• There is already an implementation of the interface and its builder in io.openliberty.mcp.internal.content, locate that first
• Add a Serializer, following the example in TextContentImpl
• Add unit tests for creation, any error cases and serialization to ToolContents
• Add FAT test for a tool returning an instance of each type