Add support for signing or checksums of elements to confirm validity

[cookeac]

Requested at the meeting on 4 May 2022. Related to #27

Be able to determine if the data associated with features has changed (implement digital signing or checksums).

Noting that data may be stored in databases and reconstituted. There are ways to do this nevertheless (if useful).

For instance the proposed JWS/CT (JSON Web Signature/Clear Text) specification combines JWS with canonical formatting of JSON (producing JSON in a perfectly standardised format that allows comparison).

Before we implement this (it would be easy to specify, more work to build into implementations), we should discuss the use cases further.

[writchie7]

I suppose this would really be beneficial if there was a single repository of maps where everyone had access to them....... I suspect we are still a couple of years away from such a repository