Add cargo xtask clippy (#8722)

This PR sets up a `cargo xtask clippy` command for running `cargo
clippy` with our defined set of options.

The intent is to make this easier to manage as we start enabling more
Clippy rules.

Release Notes:

- N/A

.cargo/ci-config.toml

@@ -10,3 +10,6 @@
 # in one spot, that's going to trigger a rebuild of all of the artifacts. Using ci-config.toml we can define these overrides for CI in one spot and not worry about it.
 [build]
 rustflags = ["-D", "warnings"]
+
+[alias]
+xtask = "run --package xtask --"

.cargo/config.toml

@@ -1,3 +1,6 @@
 [build]
 # v0 mangling scheme provides more detailed backtraces around closures
 rustflags = ["-C", "symbol-mangling-version=v0"]
+
+[alias]
+xtask = "run --package xtask --"