SDK should not require http-related packages

[Nevay]

Is your feature request related to a problem? Currently composer require open-telemetry/sdk requires installation of http-related packages.

$ composer require open-telemetry/sdk
open-telemetry/sdk 0.0.13 requires php-http/async-client-implementation ^1.0 -> could not be found in any version, but the following packages provide it: [...]
open-telemetry/sdk 0.0.13 requires psr/http-factory-implementation ^1.0 -> could not be found in any version, but the following packages provide it: [...]

Describe the solution you'd like It should be possible to install open-telemetry/sdk without installing http-related packages.