[Feature] Allow getting and setting of indexed arguments / topic filters within handlers

首先跟踪 issue 中引用的 graph-ts handler 和生成的 datasource API，然后确定索引化 topic 过滤器在 handler 运行时是如何表示的。完成的标准是：handlers 可以获取 datasource 的 event handlers，读取 topic 值，并更新和保存这些值，同时相关测试覆盖该行为。

Description

The new indexed arguments filter is a big step forward in reducing indexing times when sifting through a large amount of events. This feature could be even more powerful as a dynamic datasource, similar to how templates operate. Ideally we could set the topic filter for a specific datasource at handler runtime. Alternatively, if we could specify templates with arguments, we could also create a new datasource and specify the topic.

Without understanding how the implementation actually works, being able to update an existing datasource seems like a more elegant solution (and a better solution for my use case). Three things would need to be exposed by graph-ts: Get Datasource, Read Topic Values, Set Topic Values. The API could look something like this:

// Since datasource names should be unique, these can be auto generated
import { DatasourceName } from '../generated'
import { Event } from '../generated/Contract/Contract'

export function handleEvent(event: Contract) {
  const eventHandlers = DatasourceName.getEventHandlers();
  const transferHandler = eventHandlers.filter(event => event.abi === 'Transfer(indexed address,indexed 
  address,uint256)');
  const topics = transferHandler.topics;
  transferHandler.topics = topics.concat([event.address.toHexString()]);
  transferHandler.save();
}

Are you aware of any blockers that must be resolved before implementing this feature? If so, which? Link to any relevant GitHub issues.

No response

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