feat: add the create-identity action

[wood-push-melon]

This pull request aims to add a juju action create-identity. The typical usage will be:

$ juju run identity-platform-admin-ui/0 create-identity schema={schema} password={password} traits.email={email}

Note:

• traits parameter can accept multiple trait fields based on the user's identity schema
• the integration with an SMTP integrator will be addressed in another PR. The create-identity action will not be functional until that PR is merged.