Keep users funded

[karl-dz]

Users either onboard themselves via Passport and Sentinel flow or are allowed onto the DoubleZero Ledger network via DZF representative.

We should only fund keys that are rent-exempt (an account that has at least 128 * 6960 lamports).

Unlike contributors, users will have at most 2 User accounts it needs to fund for rent exemption. So we should only consider making sure the user balances are topped off with 128 * 6960 + X lamports, where X is some value between 20k and 50k (for 4 to 10 transactions) per funding round