Configure proxy from config

Hi! I was wondering if it would be possible to support account-level proxy configuration in Himalaya.

the use case is having multiple accounts where each account may independently use a proxy or fall back to the current ProxySystem behavior.

making minor modifications to io-email, io-imap and io-jmap it could be possible...

e.g:

[accounts.example]
# proxy without auth
proxy.url = "..."

[accounts.example2]
# proxy with auth
proxy.url = "..."
proxy.user = "my-user"
proxy.pass.command = "pass show example2/proxy"