Add xtask for finding crates with missing licenses (#11776)

This PR adds a new `cargo xtask licenses` command for finding crates
with missing license files.

A number of crates were uncovered that were missing a license file, and
have had the appropriate license file added.

Release Notes:

- N/A

tooling/xtask/src/workspace.rs

@@ -0,0 +1,17 @@
+use std::fs;
+
+use anyhow::{anyhow, Result};
+use cargo_toml::{Manifest, Workspace};
+use toml;
+
+/// Returns the Cargo workspace.
+pub fn load_workspace() -> Result<Workspace> {
+    let workspace_cargo_toml = fs::read_to_string("Cargo.toml")?;
+    let workspace_cargo_toml: Manifest = toml::from_str(&workspace_cargo_toml)?;
+
+    let workspace = workspace_cargo_toml
+        .workspace
+        .ok_or_else(|| anyhow!("top-level Cargo.toml is not a Cargo workspace"))?;
+
+    Ok(workspace)
+}