Serve CNAME records from config file

[benfrancis]

I've been trying to reconfigure the webthings.io registration servers to use Amazon SES as an outgoing mail server instead of GMail, as what I had hoped would be an easier alternative to #100 once GMail stops supporting username & password authentication.

I've discovered that in order to verify a domain with Amazon SES you need to be able to set at least CNAME, TXT and MX records on the DNS server. As far as I can tell it's possible to statically configure TXT and MX records for the registration server in config.toml, but not currently CNAME records.

In order to verify the domain with Amazon SES we therefore need to modify the source code to enable us to manually configure CNAME records in config.toml as well. A mutual friend tells me this shouldn't be too hard, but I need a Rust developer to help.

This is somewhat time sensitive because GMail will stop supporting username & password authentication at the end of this month.

[benfrancis]

This might be a good place to start for where to look

registration_server/src/pdns.rs

Lines 256 to 257 in 697e7b1

	fn build_txt_response(
	qname: &str,