Add an option to merge interceptors arrays

[enkot]

Describe the feature

Currently calling $fetch.create() and then passing interceptors when calling $fetch() will override the interceptors from $fetch.create().
I suggest to add an option to merge them. It will be useful for implementing global logging or modifying global headers etc.
Example:

const $api = $fetch.create({
  baseURL: '/api',
  mergeInterceptors: true,
  onRequest: [
    ({ request }) => { console.log('[global log]', request) }
  ]
})

$api('/endpoint', {
  onRequest: [
    () => { console.log('[local log]', request)  }
  ]
})

The result:

[global log] /endpoint
[local log] /endpoint

Previous issue - #319

Additional information

• Would you be willing to help implement this feature?

[enkot]

Closed this in favor of #435