chore: add Cargo.toml metadata (description, license, rust-version)

- All crates: rust-version = 1.70, license = MIT
- Root + example-plugin: publish = false (not intended for crates.io)
- plugin-sdk: proper description for potential future publication

plugins/example-plugin/Cargo.toml

@@ -3,6 +3,10 @@ name = "showen-example-plugin"
 version = "0.1.0"
 authors = ["showen"]
 edition = "2021"
+rust-version = "1.70"
+description = "Example dynamic plugin for ShowenV2"
+license = "MIT"
+publish = false
 
 [lib]
 crate-type = ["cdylib"]