feat: register custom service method

[tcm390]

related: #2315

Currently, when someone want to create a new service, they must add new type as string to enum ServiceType in packages/core/src/types.

For example, right now, WebSearchService is defined in a community plugin, not in the core package. As a result, there's no ServiceType.WEB_SEARCH nor IWebSearchService in core. This makes it impossible to write code like:

import { IWebSearchService, ServiceType } from "@elizaos/core";

const websearchService = runtime.getService<IWebSearchService>(ServiceType.WEB_SEARCH);

outside the plugin, because IWebSearchService and WEB_SEARCH only exist inside the websearch plugin.

To work around this, I currently have to add WEB_SEARCH and IWebSearchService to core’s types.ts file. That defeats the purpose of having a decoupled, community-managed plugin.

This PR aims to resolve that by allowing external code to create the custom service without needing to modify or depend on core’s enums and interfaces

For example, if I need to call the web search service from the image plugin, I could do:

const latestNews = await runtime.callServiceMethod(
            "web_search",
            "search",
            {
                query: "latest news on AI Agents",
                limit: 5,
                includeImages: true,
            }
);