feat: add surfnet_offlineAccount RPC method

[CanardMandarin]

This PR aims to fix #563 by adding a new cheatcode that prevents downloading an account and accounts its owns.

Half of the work was already done thanks to the existing closed_accounts logic.
I just renamed it blocked_accounts to be more generic.

/// Tracks accounts that have been explicitly closed by the user.
/// These accounts will not be fetched from mainnet even if they don't exist in the local cache.
pub closed_accounts: HashSet<Pubkey>,

The filtering happens at fetch time in SurfnetSvmLocker::filter_downloaded_account_result that I call in get_account_local_then_remote and get_multiple_accounts_with_remote_fallback.

[MicaiahReid]

Awesome, thank you @CanardMandarin!