RFE: systemd units able to specify contents:source:data like files section

Feature Request

With the files section we have:

* **_contents_** (object): options related to the contents of the file.
    * **_compression_** (string): the type of compression used on the contents (null or gzip). Compression cannot be used with S3.
    * **_source_** (string): the URL of the file contents. Supported schemes are `http`, `https`, `tftp`, `s3`, and [`data`][rfc2397].

But with the systemd unit section we only have:

* **_contents_** (string): the contents of the unit.

This means that I can't use a single way to define file contents for files vs systemd units. I end up with doing something like:

#!/bin/bash
SCRIPT_CONTENTS=$(base64 --wrap 0 public-ipv4.sh)
SYSTEMD_UNIT_CONTENTS=$(sed 's|$|\\\\n|g' < issuegen-public-ipv4.service | tr -d '\n')
sed -e "s|SYSTEMD_UNIT_CONTENTS|${SYSTEMD_UNIT_CONTENTS}|" \
    -e "s|SCRIPT_CONTENTS|${SCRIPT_CONTENTS}|" < config.ign.in > config.ign

where in one case I can use base64 and the other I have to replace newlines with \n.

Compare the files section's contents.source handling with the systemd unit section's contents field, starting with the configuration schema and the code that processes both forms. Done means systemd units can accept contents.source with the data scheme in the same way as files, with behavior covered by the repository's relevant tests.