Support for Workers KV Store

[sshymko]

API endpoints of Workers, particularly the Workers KV store, are not presently covered by the SDK.

PHP backends should be able to read from and write to the Workers KV persistent key-value storage. Ideally, bulk writes should be also supported.

[dumityty]

I was looking for this as well and looks like there's already a PR open for it: https://github.com/cloudflare/cloudflare-php/pull/139

Seems to be in progress though it's got majority of the endpoints already implemented.