feat: fetchSetup config support promise

[zce]

This PR will...

• support promise in fetchSetup config

const hls = new Hls({
  progressive: true,
  async fetchSetup (ctx, init) {
    const signature = await sign(ctx.url)
    init.headers = { ...init.headers, 'x-signature': signature }
    return new Request(ctx.url, init)
  }
})

Why is this Pull Request needed?

This will be more friendly to users using progressive mode.
Currently only xhrSetup supports promises

Checklist

• changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• API or design changes are documented in API.md