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

plugin-sdk/Cargo.toml

@@ -3,7 +3,9 @@ name = "showen-plugin-sdk"
 version = "0.2.0"
 authors = ["showen"]
 edition = "2021"
+rust-version = "1.70"
 description = "SDK for building ShowenV2 dynamic plugins"
+license = "MIT"
 
 [dependencies]
 serde = { version = "1", features = ["derive"] }