Set a plugin integration connection

Configures a Plugin integration connection, possibly resulting in a change of the corresponding push button. This operation notifies Productboard that the 3rd party system tried (or is still trying, depending on the payload type) to establish a connection between Productboard feature and some entity in the other system. There are currently three possible outcomes of such an attempt:

1. Connected: the connection has been established and a push button corresponding to the feature in related Plugin integration should change to represent the connection. This happens when the connection with state connected is received in the request body.
2. Error: the connection could not be established and a push button corresponding to the feature in related Plugin integration should change to show the error. This happens connection with state error is received in the request body.
3. In progress: the connection is still being created (possibly asynchronously) and a push button corresponding to the feature in related Plugin integration should display an "in progress" state. This happens when the state progress is received in the request body.

There is an additional state a connection can be in which does not directly map to an outcome of a push event, the Initial state. This is the default state for all connections and has the semantics of "a connection does not currently exist for given feature". The Initial state is, however, a fully valid type from the API's perspective and it is thus possible to set a connection to the Initial state using this endpoint. The effect of that is exactly the same as if the Delete a plugin integration connection endpoint was called instead.

Note that this is the endpoint to be used when the 3rd party system processes feature push events asynchronously. In that case the flow would be similar to this:

1. A feature is pushed from Productboard
2. The 3rd party schedules a task to be processed asynchronously and returns an "in progress" response. This makes Producboard render an "in progress" state on the corresponding push button.
3. Once the 3rd party finishes processing of the task, this endpoint is called to update the connection (and by extension, the state of said push button) to either a success or a failure.