Resource templates should support handlers

Expected Behavior

There should be a ResourceTemplateSpecification, similar to (A)syncResourceSpecification, so that we can have access to an exchange and handle requests properly.

Current Behavior

Currently unlike resources or prompts, there's no way to define a handler for resource templates. The same spec is used in all contexts, but doesn't allow supplying a handler.

Context

This is a serious limitation which defeats the concept of resource templates. For example, we would like to implement something like this in Micronaut MCP:

@ResourceTemplate(uriTemplate = "file://my-resource/{item}", description="A templated resource")
    String templated(String item) {
        return "Hello, " + item;
    }

That is, the content of the template depends on the URI parameters. Today there's no way to do this because the only thing we can do is register resource templates, but no handlers. Therefore, in order to serve the resources, we have to register all of them as regular resources, which makes the templating redundant. This also assumes that we can know in advance all the combinations of valid path elements values, which is obviously not possible.

首先定位现有的 (A)syncResourceSpecification，以及资源和 prompts 使用的注册路径。跟踪 resource-template 请求的处理方式，然后定义一个具备 handler 能力的 specification 必须为 URI 参数公开哪些内容。完成标准是：模板化资源能够解析请求参数并提供内容，而无需预先注册每个 URI。