ddns-scripts: add support for Google Cloud DNS

ddns-scripts: add support for Google Cloud DNS

The implementation uses a GCP service account. The user is expected to
create and secure a service account and generate a private key. The
"password" field can contain the key inline or be a file path pointing
to the key file on the router.

The GCP project name and Cloud DNS ManagedZone must also be provided.
These are taken as form-urlencoded key-value pairs in param_enc. The TTL
can optionally be supplied in param_opt.

Signed-off-by: Chris Barrick <chrisbarrick@google.com>