feat: add support for configuring use_fips_sts_endpoint in sigv4 config

Using `AWS_USE_FIPS_ENDPOINT` to configure FIPS endpoint usage no longer
works (see https://github.com/prometheus/prometheus/issues/16752). This
must be done using the `use_fips_sts_endpoint` config option added in
https://github.com/prometheus/common/pull/649. It is also available in
the new sigv4 package: https://github.com/prometheus/sigv4.

Fixes #7986

Signed-off-by: Sam McBroom <sam.mcbroom@appian.com>

.github/workflows/cspell.json

@@ -417,6 +417,8 @@
     "authzed",
     "resolv",
     "finalizer",
-    "selfsigned"
+    "selfsigned",
+    "FIPS",
+    "FIPSSTS"
   ]
-}
+}