feat: add support for Guzzle 8

[GrahamCampbell]

Guzzle 8.0.0 was released on July 20th. Lots of people want to use it, and thus popular packages in the ecosystem like this one need to be updated to support it, so here I am, adding support. :)

[bshaffer]

Is there no way to use instanceof here instead using the appropriate subclass? Using method_exists seems a little hacky

[GrahamCampbell]

No, because of the need to keep Guzzle 7 support. Guzzle 7 has RequestException and Guzzle 8 and both RequestException and ResponseException. RequestException in Guzzle 7 has getResponse, but only ResponseException in Guzzle 8 has getResponse. After dropping Guzzle 7 at some point in the future, this can be replaced with an instanceof ResponseException

---

Guzzle 7.x uses this hierarchy:

. \RuntimeException
└── TransferException (implements GuzzleException)
    ├── ConnectException (implements NetworkExceptionInterface)
    └── RequestException (implements RequestExceptionInterface)
        ├── BadResponseException
        │   ├── ClientException
        │   └── ServerException
        └── TooManyRedirectsException

Guzzle 8.0 uses this hierarchy:

. \RuntimeException
└── TransferException (implements GuzzleException)
    ├── HandlerClosedException
    ├── NetworkException (implements NetworkExceptionInterface)
    │   ├── ConnectException
    │   │   └── ConnectTimeoutException
    │   └── NetworkTimeoutException
    └── RequestException (implements RequestExceptionInterface)
        └── ResponseException
            ├── BadResponseException
            │   ├── ClientException
            │   └── ServerException
            ├── ResponseTransferException
            │   └── ResponseTimeoutException
            └── TooManyRedirectsException