Prepare this adapter for Faraday 2.0

[mattbrictson]

In Faraday 2.0, adapters will be completely opt-in. As a consequence, this adapter can now have a hard dependency on net_http_persistent. If a user has opted into adding this adapter to their Gemfile, we know they will be using it.

Thus this PR can make the following changes:

1. Remove the dependency "net/http/persistent" declaration. This existed to support loading dependencies on demand, based on what adapter is being used. Since adapters are now explicitly opt-in, this is no longer needed.
2. Add faraday-net_http as an explicit dependency. The faraday-net_http_persistent adapter subclasses the net_http one. Previously the net_http adapter was part of Faraday core. As of 2.0 it is packaged as a separate gem, so we need to declare it as a dependency.
3. Change the installation instructions. All a user needs to do now is install faraday-net_http_persistent. All other dependencies will be included automatically.

[mattbrictson]

Thanks! I left off these things out of this PR because they aren't strictly related to Faraday 2.0 compatibility:

• Dropping Ruby 2.5
• Dropping net-http-persistent 3.x
• Moving development dependencies to the Gemfile
• Fixing the changelog URI

But I think those are worthwhile improvements and we should cherry-pick those from #8. As a follow-up PR.

[olleolleolle]

@mattbrictson I began taking some of these steps, so watch this space.

[olleolleolle]

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