feat: @handler support

[Lellansin]

@Provide()
@Func('')     // TODO midway/decorator allow undefined
export class IndexService implements FunctionHandler {
  @Inject()
  ctx; // context

  @Handler('index.entry')
  handler(event) {
    return event.text + this.ctx.text;
  }

  @Handler('index.list')
  getList(event) {
    return event.text + this.ctx.text;
  }
}

the @fun and @handler, can be used in same time. such as:

import { Inject, Provide, Func, Handler } from '@midwayjs/decorator';
import { FunctionHandler } from '../../../../src';

@Provide()
@Func('index.handler')
export class IndexService implements FunctionHandler {
  @Inject()
  ctx; // context

  // index.handler default method
  handler(event) {
    return event.text + this.ctx.text;
  }

  @Handler('index.list')
  getList(event) {
    return event.text + this.ctx.text;
  }
}