BUG/MINOR: acme: contact mail should be optional, don't pass ToS bool

BUG/MINOR: acme: contact mail should be optional, don't pass ToS bool

According to ACME RFC contact email is optional.
Letsencrypt used it some long time ago, but not today.
Currently HAProxy always sets the value of the contact mail to a string
that is read from the config, but if that string is not specified,
it sets %s in mailto:%s to null, which cases new account request
to fail in pebble.

Also HAProxy currently passes termsOfServiceAgreed bool to requests
that contain onlyReturnExisting, that isn't needed according to the RFC
and other ACME impls.

This patch dynamically builds the account request JSON to address that.

Can be backported to 3.2