KillSwitches: flow stage from CancellationToken

[Horusiath]

This PR introduces an extension method, that allows to produce a stage (similar in design to SharedKillSwitch) from CancellationToken:

var cts = new CancellationTokenSource();

source
.Via(cts.Token.AsFlow<string>(cancelGracefully: true))
.RunForEach(Console.WriteLine, materializer)

cancelGracefully parameter defines if calling Cancel on token should cause stream shutdown in graceful way (via Complete) or as a failure (via OperationCanceledException).

[marcpiechura]

@Horusiath api approval is missing and all tests are failing

System.InvalidCastException : Unable to cast object of type 'CancellableKillSwitchStage1[System.Int32]' to type 'Akka.Streams.Stage.IGraphStageWithMaterializedValue2[Akka.Streams.Shape,System.Object]'.
at Akka.Streams.Stage.GraphStageWithMaterializedValue2.<.ctor>b__2_0() at System.Lazy1.CreateValue()
at System.Lazy1.LazyInitValue() at Akka.Streams.Dsl.Source2.ViaMaterialized[TOut2,TMat2,TMat3](IGraph2 flow, Func3 combine)
at Akka.Streams.Tests.Dsl.FlowKillSwitchSpec.<A_CancellationToken_flow_must_complete_immediately_flows_materialized_after_switch_shutdown>b__28_0()
at Akka.Streams.TestKit.Tests.Utils.<>c__DisplayClass3_0.b__0()
at Akka.Streams.TestKit.Tests.Utils.AssertAllStagesStopped[T](AkkaSpec spec, Func`1 block, IMaterializer materializer)

[marcpiechura]

LGTM 👍