Add monitor node for graph introspection

Adds a `serialize` function on Node which can be implemented by nodes to
allow introspecting their content. This pr also adds a Monitor Node that
always caches the last value it was evaluated with.

Test Plan: -

Reviewers: 0HyperCube

Reviewed By: 0HyperCube

Pull Request: https://github.com/GraphiteEditor/Graphite/pull/1165

node-graph/gstd/Cargo.toml

@@ -11,12 +11,7 @@ license = "MIT OR Apache-2.0"
 [features]
 memoization = ["once_cell"]
 default = ["memoization"]
-gpu = [
-	"graphene-core/gpu",
-	"gpu-compiler-bin-wrapper",
-	"compilation-client",
-	"gpu-executor",
-]
+gpu = ["graphene-core/gpu", "gpu-compiler-bin-wrapper", "compilation-client", "gpu-executor"]
 vulkan = ["gpu", "vulkan-executor"]
 wgpu = ["gpu", "wgpu-executor"]
 quantization = ["autoquant"]
@@ -63,6 +58,7 @@ glam = { version = "0.22", features = ["serde"] }
 node-macro = { path = "../node-macro" }
 boxcar = "0.1.0"
 xxhash-rust = { workspace = true }
+serde_json = "1.0.96"
 
 [dependencies.serde]
 version = "1.0"