proto: fix parse of arbitrary keys in pres. syntax

Previously the `FromStr` impl for `SvcParamKey` had support for parsing
the "arbitrary key" presentation syntax where a key can be specified
"keyNNNNN", where NNNNN is the numeric value of the key type without
leading zeros. The existing code would pull out the numeric component
into a `u16` and then use the `TryFrom<u16>` impl for `SvcParamKey` to
get the key.

However, the `TryFrom<u16>` impl for `SvcParamKey` was using the IANA
service parameter keys registry to map from u16s to `SvcParamKey`.
Values 0..6 are mapped to the known key entries. The reserved range
(65280-65534) was mapped to `SvcParamKey::Key`, and 65535 was mapped to
`SvcParamKey::Key65535`. This makes sense when mapping an arbitrary u16,
but when we are parsing a "keyNNNNN" presentation syntax item, we want
to represent it as `Key(NNNNN)`, no matter if it is/isn't a registered
key.

This commit fixes this behaviour, constructing a `SvcParamKey::Key()`
entry when parsing the arbitrary key presentation syntax, avoiding
`TryFrom<u16>`.

With this change in place the two arbitrary key test vectors can be
included in the svcb test vector unit test.

[0] https://datatracker.ietf.org/doc/html/rfc9460#name-initial-contents

crates/proto/src/rr/rdata/svcb.rs

@@ -307,9 +307,7 @@ impl std::str::FromStr for SvcParamKey {
                 )))
             })?;
 
-            let key_value = u16::from_str(key_value)?;
-            let key = SvcParamKey::from(key_value);
-            Ok(key)
+            Ok(SvcParamKey::Key(u16::from_str(key_value)?))
         }
 
         let key = match s {

crates/proto/src/serialize/txt/rdata_parsers/svcb.rs

@@ -452,7 +452,7 @@ mod tests {
 
         // NOTE: In each case the test vector from the RFC was augmented with a TTL (42 in each
         //       case). The parser requires this but the test vectors do not include it.
-        let vectors: [TestVector; 6] = [
+        let vectors: [TestVector; 8] = [
             // https://datatracker.ietf.org/doc/html/rfc9460#appendix-D.1
             // Figure 2: AliasMode
             TestVector {
@@ -479,8 +479,6 @@ mod tests {
                 priority: 16,
                 params: vec![(SvcParamKey::Port, SvcParamValue::Port(53))],
             },
-            /*
-             * TODO(XXX): ParseError { kind: Message("Bad Key type or unsupported, see generic key option, e.g. key1234"), backtrack: None }
             // Figure 5: A Generic Key and Unquoted Value
             TestVector {
                 record: "example.com. 42  SVCB   1 foo.example.com. key667=hello",
@@ -503,7 +501,6 @@ mod tests {
                     SvcParamValue::Unknown(Unknown(b"hello\\210qoo".into())),
                 )],
             },
-             */
             // Figure 7: Two Quoted IPv6 Hints
             TestVector {
                 record: r#"example.com. 42  SVCB   1 foo.example.com. (ipv6hint="2001:db8::1,2001:db8::53:1")"#,