Provide the directory URL to the account cache

[joshtriplett]

This supports using and caching different accounts with different directories.

PR based on https://github.com/FlorianUekermann/rustls-acme/pull/21 to avoid conflicts.

[FlorianUekermann]

I considered doing this initially, but I couldn't come up with a reason why different accounts for different directories are useful. I already merged, but could you explain what made you want this?

[joshtriplett]

@FlorianUekermann Different directories have different standards for security and robustness. Using an account with one directory (e.g. staging) may make it inadvisable to use with another (e.g. production).

Also, users of this may have different standards for security and robustness; for instance, account keys for staging accounts aren't quite as critical to keep secret as account keys for production accounts.