FR: Support the use of a private key as a node wallet.

[jshufro]

Use case: Bob signs up for allnodes using metamask as his node wallet. Bob creates 3 minipools. Bob wants to move to his own hardware. Bob cannot find the mnemonic for his metamask wallet, but is able to export the private key.

Non-features: Bob will not be able to make additional minipools.

Ideally, the private key will be "loaded" from the cli and encrypted with the same .rocketpool/data/password as usual. However, it probably shouldn't be saved in wallet, since it isn't a wallet, and derivation of further keys (validator or other) doesn't, as I understand it, work.

The wallet abstraction will have to check if a wallet file exists and prefer that, but fall back on a private key file.