Refactor FastlyClient

Why is this change being made?

Issue: #9087 (closed)

• Extract FastlyClient into a separate class
• Add tests for FastlyClient
• Add error handling for Fastly API calls

Problem

There are two limits on API side that we reached.

We cannot send a batch with more than 1000 items to dictionary API

[fixed]

That I fixed by splitting items into smaller batches.

Each dictionary has a limit of 1000 items.

[todo]

According to documentation we can request Fastly support to increase it. (see https://docs.fastly.com/en/guides/resource-limits#vcl-and-configuration-limits, Edge dictionary items count)

Another option is to create a new dictionary and distribute items between them. But I haven't worked by Fastly API before and I don't know if it's doable and makes sense.