Add Sequencers to actor Instance (#1299)

Summary:

We want to move sequence assignment to the client side, and actor Instance is a good place for that, since:

1. we can get it through `context::Actor`;
2. there is ongoing effort to deprecate `cap::CanSend` with `context::Actor`. so `context::Actor` will be accessible in client's message sending callsites. 

This diff:

1. adds a `Struct Sequencers` which encapsulate the assignment logic.
2. adds a `fn seq_send` method to `ActorRef` show how it is used by the client.

Differential Revision: D82983004

Author: pzhan9

hyperactor/Cargo.toml

@@ -84,6 +84,7 @@ tokio-stream = { version = "0.1.17", features = ["fs", "io-util", "net", "signal
 tokio-util = { version = "0.7.15", features = ["full"] }
 tracing = { version = "0.1.41", features = ["attributes", "valuable"] }
 unicode-ident = "1.0.12"
+uuid = { version = "1.2", features = ["serde", "v4", "v5", "v6", "v7", "v8"] }
 
 [dev-dependencies]
 buck-resources = "1"