improve media key decoding logs

On my HS I observed 5 instances of keys with the following format:

* MXC bytes.
* A 0xFF byte.
* 4 bytes where the width and height are supposed to be, which are
  supposed to be 8 bytes in length.
* 3 consecutive 0xFF bytes. This means that the `content-type` and
  `content-disposition` sections both parse as the empty string, and
  there's an extra separator at the end too.
* Extra bytes, all of which were `image/png`.

The 4 bytes where the width and height are supposed to be were one of:

* 003ED000
* 003EE000
* 003EF001

Which seems to have some kind of pattern to it...

After much digging, we have absolutely no idea what could've caused
this. Cursed.

src/utils.rs

@@ -4,6 +4,7 @@ pub(crate) mod on_demand_hashmap;
 use std::{
     borrow::Cow,
     cmp, fmt,
+    fmt::Write,
     str::FromStr,
     time::{SystemTime, UNIX_EPOCH},
 };
@@ -366,6 +367,17 @@ pub(crate) fn curlify<T>(req: &http::Request<T>) -> Option<String> {
     )
 }
 
+/// Format a u8 slice as an uppercase hex string
+///
+/// The output does not contain a leading `0x` nor any non-hex characters (e.g.
+/// whitespace or commas do not appear in the output).
+pub(crate) fn u8_slice_to_hex(slice: &[u8]) -> String {
+    slice.iter().fold(String::new(), |mut acc, x| {
+        write!(acc, "{x:X}").expect("in-memory write should succeed");
+        acc
+    })
+}
+
 #[cfg(test)]
 mod tests {
     use crate::utils::dbg_truncate_str;