util: read .json files as HuJSON in UnmarshalByExt (#3634)

dns.extra_records_path and derp.paths files ending in .json were decoded
with encoding/json, so a comment or trailing comma in otherwise valid
JSON made headscale refuse to load the file. Policy files already accept
HuJSON regardless of extension. HuJSON is a superset of JSON, so
standardizing .json input the same way as .hujson keeps every valid JSON
file decoding exactly as before.

Add tests for comments and trailing commas in extra records, in generic
.json decoding and in ACL policies, and for comment markers inside
string values.

Fixes #3627

docs/ref/derp.md

@@ -55,7 +55,8 @@ derp:
 
 The DERP map offered to clients can be customized with a [dedicated YAML-configuration
 file](https://github.com/juanfont/headscale/blob/main/derp-example.yaml). The file extension picks the format: `.yaml`
-or `.yml`, or the JSON format Tailscale serves DERP maps in as `.json` or `.hujson`. This allows to modify previously
+or `.yml`, or the JSON format Tailscale serves DERP maps in as `.json` or `.hujson`. Both JSON extensions are read as
+[HuJSON](https://github.com/tailscale/hujson), so comments and trailing commas are allowed. This allows to modify previously
 loaded DERP maps fetched via URL or to offer your own, custom DERP servers to nodes.
 
 === "Remove specific DERP regions"

docs/ref/dns.md

@@ -68,6 +68,8 @@ hostname and port combination "http://hostname-in-magic-dns.myvpn.example.com:30
 
             - The `dns.extra_records_path` option in the [configuration file](configuration.md) needs to reference the
               file containing extra DNS records. Its extension picks the format: `.json`, `.hujson`, `.yaml` or `.yml`.
+              A `.json` file is read as [HuJSON](https://github.com/tailscale/hujson), so it may contain comments and
+              trailing commas.
             - Be sure to "sort keys" and produce a stable output in case you generate the JSON file with a script.
               Headscale uses a checksum to detect changes to the file and a stable output avoids unnecessary processing.