add missing valid rkey char (#1131)

by [the rkey
spec](https://atproto.com/specs/record-key#record-key-syntax):

> restricted to a subset of ASCII characters — the allowed characters
are alphanumeric (A-Za-z0-9), period, dash, underscore, colon, or tilde
(.-_:~)

tilde was missing from this check, which leads to at least [jetstream
becoming unhappy](https://github.com/bluesky-social/jetstream/issues/54)
over valid-but-uncommon rkeys.

this change aligns the check with the spec.

mst/mst_test.go

···
 	for i := 0; i < 256; i++ {
 		f.Add([]byte{byte(i)})
 	}
-	rx := regexp.MustCompile("^[a-zA-Z0-9_:.-]+$")
+	rx := regexp.MustCompile("^[a-zA-Z0-9_:.~-]+$")
 	f.Fuzz(func(t *testing.T, in []byte) {
 		s := string(in)
 		if a, b := rx.MatchString(s), keyHasAllValidChars(s); a != b {

mst/mst_util.go

···
 }
 
 // keyHasAllValidChars reports whether s matches
-// the regexp /^[a-zA-Z0-9_:.-]+$/ without using regexp,
+// the regexp /^[a-zA-Z0-9_:.~-]+$/ without using regexp,
 // which is slower.
 func keyHasAllValidChars(s string) bool {
 	if len(s) == 0 {
···
 			continue
 		}
 		switch b {
-		case '_', ':', '.', '-':
+		case '_', ':', '.', '~', '-':
 			continue
 		default:
 			return false