Spike: Support for required metadata

In a project with required metadata fields, `flagsmith feature create` always fails:

```
$ flagsmith feature create cli_max_items --value 25 --yes
Error: POST https://api-staging.flagsmith.com/api/v1/projects/6111/features/ returned 400 Bad Request: Missing required metadata field: Required int field
Think this shouldn't happen? Tell us: https://github.com/Flagsmith/flagsmith-cli/issues/new
```

`feature create` has currently no way to send metadata.

We should spot all the commands this affects, and update design documentation with metadata support

Start with the `feature create` command and reproduce the required-metadata failure shown in the issue. Inspect the other commands that create or update resources to identify which are affected, then review the existing design documentation. Done means the affected command scope is documented and the metadata-support design is updated.

索引モデルが issue の本文から書いたものです。