Updated fetch_shas utility to use the channel manifest tomls (#2840)

Updating shas was taking much longer than it used to. I've refactored
the shell script into python so to manage increased complexity. The
script now fetches the `channel-rust-*.toml` files which contains
additional sha256 values. This script still falls back to brute force
fetching of artifacts to account for any that are missing. The runtime
was reduced from ~20 min to ~80s.

```bash
python3.11 ./util/fetch_shas/fetch_shas.py
```
```
2024-09-06 08:57:22 - INFO - Fetching known sha256 data...
2024-09-06 08:57:22 - INFO - Downloading data...
2024-09-06 08:57:29 - INFO - Done.
2024-09-06 08:57:29 - INFO - Deserializing 119 tomls...
2024-09-06 08:57:36 - INFO - Done.
2024-09-06 08:57:36 - INFO - Parsing artifacts...
2024-09-06 08:57:36 - INFO - Done. Identified 52790 artifacts.
2024-09-06 08:57:36 - INFO - Checking for 81514 missing artifacts...
2024-09-06 08:58:26 - INFO - Done.
2024-09-06 08:58:29 - INFO - Downloading 404 missing artifacts...
2024-09-06 08:58:29 - INFO - Done.
2024-09-06 08:58:36 - INFO - Done. Wrote rust/known_shas.bzl
```

Note that `beta` sha256 values have been removed in this change.

util/BUILD.bazel

@@ -1,9 +1,3 @@
-sh_binary(
-    name = "fetch_shas",
-    srcs = ["fetch_shas.sh"],
-    tags = ["manual"],
-)
-
 alias(
     name = "collect_coverage",
     actual = "//util/collect_coverage",

util/fetch_shas/BUILD.bazel

@@ -0,0 +1,14 @@
+load("@rules_python//python:defs.bzl", "py_binary")
+
+py_binary(
+    name = "fetch_shas",
+    srcs = ["fetch_shas.py"],
+    data = [
+        "fetch_shas_HOST_TOOLS.txt",
+        "fetch_shas_NIGHTLY_ISO_DATES.txt",
+        "fetch_shas_TARGETS.txt",
+        "fetch_shas_TOOLS.txt",
+        "fetch_shas_VERSIONS.txt",
+    ],
+    tags = ["manual"],
+)