Add support for compound CRS as CURIEs

[balazsdukai]

Allow compound CRS in metadata.referenceSystem the form of CURIEs, in addition to the current URI syntax.

For example:

"referenceSystem": [
    "[EPSG:4326]",
    "[EPSG:3855]"
  ]

The schema allows any UPPER/lowercase letter or digit as identifier, and any UPPER/lowercase letter as authority.

However, this change breaks backwards compatibility, so it would require a new major version.

If we decide to merge this PR, the specs need to be updated too.

Fixes #181