refactor(plugins/cohere): migrate plugin to v2 API

[CorieW]

Resolves #351

What this does

• Upgrades Genkit to the latest version
• Migrates the plugin API to v2 (doc)

Testing

• Checked that the models and embedders load with the same names/prefixes on the UI.
• Checked that at least one model works consistent with pre-migration on the UI.
• Checked that at least one embedder works consistent with pre-migration on the UI.
• Using a model object, checked using a flow works consistent with pre-migration.
• Using an embedder object, checked using a flow works consistent with pre-migration.
• Using a model string ('pluginName/modelName'), checked using a flow works consistent with pre-migration.
• Using an embedder string, checked using a flow works consistent with pre-migration.
• Tested error response from embedder.
• Tested error response from model.
• Tested resolve

Differences

• When an error occurs, the error response is different for models (not for embedders):
  v1
  v2