Async operations

[simPod]

Is your feature request related to a problem? Please describe.

I'm loading several pages from graphql api and need to do it asynchronously in order to make it faster

Describe the solution you'd like

Still am not sure. Investigating, though it seems it would need a large rework.

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I'm calling gql api using PSR Request through async client. Trying to do the same with Sailor is not possible rn.

E.g. Symfony HTTP client has async requests by default but Response makes it blocking since it reads status code and contents right after calling client.

https://github.com/spawnia/sailor/blob/44678a8db4dd864c98c6bb36c3af999465cc8bf2/src/Response.php#L38-L47

[spawnia]

I am not planning to work on this any time soon, but am open to reviewing possible desgins or a merge request. Also, I am not afraid of breaking changes, given this library is pre-v1.