feat(wasm): combine config and json_config fields

[flrgh]

Summary

This change combines the config and json_config fields for Wasm filters.

Before, JSON configuration lived in the json_config attribute, and config could only hold strings:

{
  filters = {
    {
      name = "my-filter-with-string-config",
      config = "a=1 b=2",
    },
    {
      name = "my-filter-with-json-config",
      json_config = {
        a = 1,
        b = 2,
      }
    },
  }
}

Now, both string and JSON configurations live in config:

{
  filters = {
    {
      name = "my-filter-with-string-config",
      config = "a=1 b=2",
    },
    {
      name = "my-filter-with-json-config",
      config = {
        a = 1,
        b = 2,
      }
    },
  }
}

The json_config field/attribute was introduced recently and is unreleased, so this change does not break any existing API.

Note: filters may only use JSON in config if they have supplied a JSON schema in a {{filter}}.meta.json file. Filters without a JSON schema may still accept JSON configuration, but it must be serialized, and it will not be validated in any way by the admin API:

{
  filters = {
    {
      name = "my-filter-with-json-config-and-schema",
      config = {
        a = 1,
        b = 2,
      }
    },
    {
      name = "my-filter-with-json-config-but-no-schema",
      config = [[{
        "a": 1,
        "b": 2
      }]]
    },
  }
}

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Full changelog

• implement json_schema.default fallback schema for dynamic JSON subschemas
• Remove filter_chain.filters[].config and rename filter_chain.filters[].json_config to filter_chain.filters[].config
• Create a default/fallback schema for filter_chain.filters[].config of { "type": [ "string", "null" ] }

Issue reference

KAG-2707

[flrgh]

Reviewers: we've judged filters that require a JSON-encoded string to be an uncommon use case, compared to, say, an object or an array, which will still work perfectly fine.

[flrgh]

note: changes to Wasm have been given special approval for merge after feature freeze, due to the feature's self-contained nature and current beta/tech preview/unstable status.

[flrgh]

just rebased