feat(build-analysis): emit rebuild reason log entry (#16203)

### What does this PR try to resolve?

This implements a basic emission for rebuild reasons.

It reuses the `DirtyReason` enum in the log messages.
Hence the serialization change.

Part of #15844

### How to test and review this PR?

The schema is not final, subject to change,
but at least this PR unblocks us the exploration of `cargo report`
commands.

See tests in dirty_reason.rs for the shape of the JSON message.

#### Schema decisions

We may need to review this before stabilization:

* `FileTime`: serialize as f64 milliseconds
  Millisecond precision should be sufficient for mtime comparison,
  if not we can change to nanosecond
* `Checksum`: serializing to Display format
  (e.g., "sha256=abc123...")
* `StaleItem`: with internally tagged format, for example
  ```
  {"stale-item":"missing-file","path":"src/lib.rs"}
  {"stale-item":"changed-file","reference_mtime":"...", ...}
  ```
* `FsStatus`: with internally tagged format, for example
  ```
  {"fs-status":"stale-item","type":"missing-file","path":"..."}
  ```
  The `UpToDate` variant is skipped
  since it's not relevant for rebuild reason logging.

Author: epage

src/cargo/core/compiler/fingerprint/dep_info.rs

@@ -18,6 +18,7 @@ use anyhow::bail;
 use cargo_util::ProcessBuilder;
 use cargo_util::Sha256;
 use cargo_util::paths;
+use serde::Serialize;
 
 use crate::CARGO_ENV;
 use crate::CargoResult;
@@ -660,6 +661,15 @@ impl fmt::Display for Checksum {
     }
 }
 
+impl Serialize for Checksum {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        serializer.serialize_str(&self.to_string())
+    }
+}
+
 #[derive(Debug, thiserror::Error)]
 pub enum InvalidChecksum {
     #[error("algorithm portion incorrect, expected `sha256`, or `blake3`")]