acme: ability to specify the exact domains to use for TLS as well as wildcards

[komuw]

• Currently if you have a config like;

  domain := "*.example.com"
  o := config.LetsEncryptOpts(domain, ...)
  _ = server.Run(mux, o)

  Any random bot that makes a request to any subdomain will lead to a certificate for that subdomain been created. From small time experience, within no time; you end up with hundreds of subdomains just because bots are requesting for them.

  > ls -lsha .config/ong_acme/ | wc -l
  242 # 242 subdomains

Maybe we should expand the server configs; https://github.com/komuw/ong/blob/7a7804416d8fcea006bdd65350c8353e3e9bfe4d/config/config.go#L735-L736 so that it takes in an option like;

// HostPolicy specifies which host names the Manager is allowed to respond to.
// It returns a non-nil error if the host should be rejected.
// The returned error is accessible via tls.Conn.Handshake and its callers.
// See Manager's HostPolicy field and GetCertificate method docs for more details.
type HostPolicy func(ctx context.Context, host string) error

Also see; https://github.com/golang/crypto/blob/dbb6ec16ecef7a66638d8514be54b13660551b0a/acme/autocert/autocert.go#L68-L88 which allows you to provide a list of domains and subdomains that you allow certificates for.

[komuw]

Alternatively, a case where we replace the argument domain with domains []string. If any of the domains in the slice has a * then it is interpreted as per; https://github.com/komuw/ong/blob/7a7804416d8fcea006bdd65350c8353e3e9bfe4d/internal/acme/acme.go#L44-L69 We could even ban the mixing of domains with * and any other domains

domains := []string{"*.example.com", "api.example.com"} // would be banned.
o := config.LetsEncryptOpts(domains, ...)
_ = server.Run(mux, o)

domains := []string{"login.example.com", "api.example.com"} // this is allowed.
o := config.LetsEncryptOpts(domains, ...)
_ = server.Run(mux, o)

domains := []string{"*.example.com"} // this is allowed.
o := config.LetsEncryptOpts(domains, ...)
_ = server.Run(mux, o)

[komuw]

We could retain the argument called domain but only use it in middlewares. Then create a new argument called tlsHosts for use in the server.