Create default directory if it doesn't exist

[AgeManning]

Issue Addressed

I'm trying to run Anchor on unstable via anchor node on a fresh build.

It fails because:

2025-03-20T02:24:41.373883Z  INFO client: Starting the Anchor client beacon_nodes="[\"http://localhost:5052/\"]" execution_nodes="[\"http://localhost:8545/\", \"ws://localhost:8546/\"]" data_dir="\"/home/age/.anchor/holesky\""
2025-03-20T02:24:41.374907Z  INFO client: Creating private key path=/home/age/.anchor/holesky/key.pem
2025-03-20T02:24:41.414674Z ERROR anchor: Failed to start Anchor reason="Unable to write private key: Output(Os { code: 2, kind: NotFound, message: \"No such file or directory\" })"

The default path doesn't exist.

This PR will create the directory if it doesn't exist allowing the node to start part this point.