Explicitly version schema

[yongrenjie]

• renames schema.json -> schema/popgetter_0.1.0.json
• adds a version field to the JSON itself
• updates README

Still not sure about

• This is obviously still somewhat error-prone. How can we enforce version consistency? Ideally we would like to have a single (global) variable:

const POPGETTER_SUPPORTED_VERSION: &str = "0.1.0"

and then import the metadata like

import_types!(concat!("schema/popgetter_", POPGETTER_SUPPORTED_VERSION, ".json"));

but macro-in-macro doesn't work because import_types! only accepts a single string literal (as far as I can tell)

It would then also be good to have a test to read in the JSON and check that the version field is indeed equal to POPGETTER_SUPPORTED_VERSION, because typify doesn't give us access to properties apart from the type definitions.

To try to minimise the chances of inconsistencies here...

I've changed the popgetter-export-schema upstream command to always use its __version__ (see https://github.com/Urban-Analytics-Technology-Platform/popgetter/pull/80)

and also documented it in this repo's README.

[yongrenjie]

^ Rebased off main.