Improve account management

[martriay]

Today there's an implied 1:1 relationship between accounts, singers, and private keys. I think we should design better around an alias system for accounts that also supports multi-signer or different signing schemes, and signers decoupled from private keys to allow rotation.

Decoupling accounts from signers can also help us reuse signers across networks, and set defaults like max_fee for an improved UX, besides fixing awkwardness like passing a private key alias to send or declare.