feat: cache derived keys for faster keying (#10114)

Currently `DeriveKey` is called every time that a secret must be encoded/decoded. Since this function is deterministic, its result can be cached to allow a 250x speedup (the original took less than half a microsecond, so this more of a micro-optimization...).

```
go test -bench=.
goos: linux
goarch: amd64
pkg: forgejo.org/modules/keying
cpu: Intel(R) Core(TM) Ultra 5 125H
BenchmarkExpandPRK-18            2071627               564.2 ns/op
BenchmarkExpandPRKOnce-18       541438192                2.206 ns/op
PASS
ok      forgejo.org/modules/keying      2.369s
```

## Other changes

- Since the keys can be constructed once, it simplifies a bit the callsites (`keying.TOTP.Encrypt(...)` instead of `keying.DeriveKey(keying.ContextTOTP).Encrypt(...)`)
- All `Encrypt`/`Decrypt` calls will panic forever if called before `Init` has been called (current it panics as long as `Init` has not been called)
- Calling `Init` twice with different keys will trigger a panic (currently racy)
- Calling `Decrypt` with a short ciphertext does not panic anymore (like when calling with long-enough garbage)

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/10114
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: oliverpool <git@olivier.pfad.fr>
Co-committed-by: oliverpool <git@olivier.pfad.fr>

services/task/task.go

@@ -84,7 +84,7 @@ func CreateMigrateTask(ctx context.Context, doer, u *user_model.User, opts base.
 			return err
 		}
 
-		key := keying.DeriveKey(keying.ContextMigrateTask)
+		key := keying.MigrateTask
 
 		opts.CloneAddrEncrypted = base64.RawStdEncoding.EncodeToString(key.Encrypt([]byte(opts.CloneAddr), keying.ColumnAndJSONSelectorAndID("payload_content", "clone_addr_encrypted", task.ID)))
 		opts.CloneAddr = util.SanitizeCredentialURLs(opts.CloneAddr)