Add call_api() function to call plugins

[sweep-ai[bot]]

This PR adds a call_api() function to the Plugins class to call an operation in an active plugin.

The function takes the following arguments:

• plugin_name: The name of the plugin
• operation_id: The ID of the operation to call
• parameters: The parameters to pass to the operation

It gets the PluginObject for the specified plugin, gets the operation details, and calls the operation. It returns the response from the API call.

This allows calling plugins directly from the Plugins class, in addition to using the apply_plugins decorator.

Fixes #122.