Add disable affinity feature

[nimf]

Before this PR there was no option to ignore affinity key in the message. But in some cases upper layer may want that for some calls.

This PR allows temporarily disabling affinity either via call option:

stub.withOption(GcpManagedChannel.DISABLE_AFFINITY_KEY, true).someRpcMethod(...)

or via context:

Context
    .current()
    .withValue(GcpManagedChannel.DISABLE_AFFINITY_CTX_KEY, true)
    .run(() -> {
        stub.someRpcMethod(...)
    })

[nimf]

/cc @arpan14