cmd/utils: change blssync.JWTSecretFlag to DirectoryFlag

[MariusVanDerWijden]

closes #30304

We already use DirectoryFlag for authrpc.jwtsecret which expands the tilde, so this should work out of the box

[namiloh]

A directory flag does not accept an explicit file, iirc from when I looked at this. Please provide commandline example showing that it indeed works as expected.

[MariusVanDerWijden]

go run ./cmd/blsync --sepolia --beacon.api=https://xxx.io --beacon.checkpoint=$(curl -X 'GET' -
H 'accept: application/json' 'https://xxx.io/eth/v1/beacon/headers/finalized' | jq -r ".data.root") --blsync.engine.api=http://localhost:8551 --blsync.jwtsecr
et=~/.ethereum/sepolia/geth/jwtsecret
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
curl: (7) Failed to connect to xxx.io port 443 after 912 ms: Could not connect to server
INFO [11-05|10:59:24.464] Loaded JWT secret file                   path=/home/matematik/.ethereum/sepolia/geth/jwtsecret crc32=0x865194cd
WARN [11-05|10:59:25.119] Beacon API request failed                type=sync.ReqCheckpointData reqid=1 err="Get \"https://xxx.io/eth/v1/beacon/light_client/bootstrap/0x456e85f5608afab3465a0580bff8572255f6d97af0c5f939e3f7536b5edb2d3f\": dial tcp 47.240.5.85:443: connect: connection refused"
WARN [11-05|10:59:25.129] Head event stream error                  err="error creating event subscription: Get \"https://xxx.io/eth/v1/events?topics=head&topics=light_client_finality_update&topics=light_client_optimistic_update\": dial tcp 47.240.5.85:443: connect: connection refused"
^Csignal: interrupt

(host is not responding, but it correctly resolves the secret)

[namiloh]

Neat!

[holiman]

LGTM