feat(unstable): Added -Zbuild-dir-new-layout unstable feature (#15848)

### What does this PR try to resolve?

Added `-Zbuild-dir-new-layout` unstable feature as part of #15010

I will open a follow up PR with an initial implementation as mentioned
[here](#15010 (comment))

r? @weihanglo

Author: weihanglo

src/cargo/core/features.rs

@@ -845,6 +845,7 @@ unstable_cli_options!(
     binary_dep_depinfo: bool = ("Track changes to dependency artifacts"),
     bindeps: bool = ("Allow Cargo packages to depend on bin, cdylib, and staticlib crates, and use the artifacts built by those crates"),
     build_analysis: bool = ("Record and persist build metrics across runs, with commands to query past builds."),
+    build_dir_new_layout: bool = ("Use the new build-dir filesystem layout"),
     #[serde(deserialize_with = "deserialize_comma_separated_list")]
     build_std: Option<Vec<String>>  = ("Enable Cargo to compile the standard library itself as part of a crate graph compilation"),
     #[serde(deserialize_with = "deserialize_comma_separated_list")]
@@ -1365,6 +1366,7 @@ impl CliUnstable {
             "binary-dep-depinfo" => self.binary_dep_depinfo = parse_empty(k, v)?,
             "bindeps" => self.bindeps = parse_empty(k, v)?,
             "build-analysis" => self.build_analysis = parse_empty(k, v)?,
+            "build-dir-new-layout" => self.build_dir_new_layout = parse_empty(k, v)?,
             "build-std" => self.build_std = Some(parse_list(v)),
             "build-std-features" => self.build_std_features = Some(parse_list(v)),
             "cargo-lints" => self.cargo_lints = parse_empty(k, v)?,

src/doc/src/reference/unstable.md

@@ -80,6 +80,7 @@ Each new feature described below should explain how to use it.
     * [feature-unification](#feature-unification) --- Enable new feature unification modes in workspaces
 * Output behavior
     * [artifact-dir](#artifact-dir) --- Adds a directory where artifacts are copied to.
+    * [build-dir-new-layout](#build-dir-new-layout) --- Enables the new build-dir filesystem layout
     * [Different binary name](#different-binary-name) --- Assign a name to the built binary that is separate from the crate name.
     * [root-dir](#root-dir) --- Controls the root directory relative to which paths are printed
 * Compile behavior
@@ -1939,6 +1940,14 @@ The `-Zbuild-analysis` feature records and persists detailed build metrics
 enabled = true
 ```
 
+## build-dir-new-layout
+
+* Tracking Issue: [#15010](https://github.com/rust-lang/cargo/issues/15010)
+
+Enables the new build-dir filesystem layout.
+This layout change unblocks work towards caching and locking improvements.
+
+
 # Stabilized and removed features
 
 ## Compile progress