pki: T5886: add support for ACME protocol (LetsEncrypt)

The "idea" of this PR is to add new CLI nodes under the pki subsystem to
activate ACME for any given certificate.

vyos@vyos# set pki certificate NAME acme
Possible completions:
+ domain-name Domain Name

email                Email address to associate with certificate
listen-address       Local IPv4 addresses to listen on
rsa-key-size         Size of the RSA key (default: 2048)
url                  Remote URL (default:
                     https://acme-v02.api.letsencrypt.org/directory)

Users choose if the CLI based custom certificates are used

set pki certificate EXAMPLE acme certificate <base64>

or if it should be generated via ACME.

The ACME server URL defaults to LetsEncrypt but can be changed to their staging
API for testing to not get blacklisted.

set pki certificate EXAMPLE acme url https://acme-staging-v02.api.letsencrypt.org/directory

Certificate retrieval has a certbot --dry-run stage in verify() to see if it
can be generated.

After successful generation, the certificate is stored in under
/config/auth/letsencrypt. Once a certificate is referenced in the CLI (e.g. set
interfaces ethernet eth0 eapol certificate EXAMPLE) we call
vyos.config.get_config_dict() which will (if with_pki=True is set) blend in the
base64 encoded certificate into the JSON data structure normally used when
using a certificate set by the CLI.

Using this "design" does not need any change to any other code referencing the
PKI system, as the base64 encoded certificate is already there.

certbot renewal will call the PKI python script to trigger dependency updates.