Add dns_gcloud_rest for Google Cloud DNS

This script is an alternative to the existing dns_gcloud script but
aimed at embeded environments. The existing dns_gcloud script depends
on the gcloud CLI, which is over 60 MB on ARM and over 85 MB on x86_64.
In many embeded environments, this dependency is too large to install.

This new script, dns_gcloud_rest, interfaces with Google Cloud DNS
using the REST API. It implements the service account auth flow and
requires the user to provision a service account with permission to
manage DNS zones.

The Google Cloud REST API uses JSON and this script uses regex to
extract data from the response bodies. I have done my best to implement
robust patterns, but regex is fundamentally not able to parse JSON. The
response handling is therefore sensitive to non-semantic details of the
response bodies, like newline placement. That said, some responses from
GCP are minified and some are pretty; this script handles responses in
both formats adequately.

This script is ported from a similar script I contributed the the
[OpenWRT DDNS scripts][1]. OpenWRT provides tools for JSON parsing and
formatting, which makes that version of the script more robust and
succinct than this verion.

I have tested the script in several environments to ensure portability:
- Bash 3.2.57 with macOS coreutils on macOS
- Bash 3.2.57 with GNU coreutils on macOS
- busybox 1.37.0 on OpenWRT

[1]: https://github.com/cbarrick/openwrt_packages/blob/ddns-gcp/net/ddns-scripts/files/usr/lib/ddns/update_gcp_v1.sh

Author: cbarrick