Add "schemas" to extension-schema.json

[jpmckinney]

This issue is motivated by the following:

1. It is not possible for a service to determine which core schema files an extension patches without testing all three possible URLs (http…/release-schema.json, etc.)
2. We would like it to be possible for a service to determine which core schema files an extension patches without doing this sort of URL discovery
3. JSON Schema Patch files in extensions are optional (see https://github.com/open-contracting/standard_extension_template/issues/29)
4. We would like CoVE to display an appropriate error if an explicitly expected patch file is missing, but not error otherwise

As background:

• In https://github.com/open-contracting/ocds-extensions/issues/59, I proposed adding a schemas field to extension.json similar to the codelists field.
• In https://github.com/open-contracting/standard_extension_template/issues/29#issuecomment-391134423, I proposed that CoVE's behavior should be to error if it tries to fetch a schema listed in schemas and fails, but report no error if there are no schemas specified and it fails to optimistically retrieve the release schema.

If this issue is agreed, I can merge this and then use the local:extension_json task it contains to update all extension repositories, as I've done in the past for other changes to all extensions.

Requesting review from one of @duncandewhurst @kindly @timgdavies

[jpmckinney]

Related issue: https://github.com/OpenDataServices/cove/issues/1024

[jpmckinney]

@duncandewhurst @kindly @timgdavies As this may have an impact on the extension explorer, could one of you review by end of next week? cc @robredpath

[duncandewhurst]

Discussed with @kindly and this is OK to be merged.