Context Implementation - Global Operations - Detatch Context

[bobstrecansky]

Because we are implicitly implementing the Context feature for OpenTelemetry-php, we need to make sure that we have the global operations that we are expected to have for the API. These operations are listed here.

This ticket specifically covers the ability to Detach from the current Context. this is defined here

Success criteria:

The API MUST accept the following parameters:

• A Token that was returned by a previous call to attach a Context.

The API MAY return a value used to check whether the operation was successful or not.

[bobstrecansky]

This will be solved with https://github.com/open-telemetry/opentelemetry-php/pull/111