Allow an option to lock the conversation to a single thread in API channels

[jmelati]

Is your feature or enhancement related to a problem? Please describe.

In API channel, I don´t want who my agents create new conversations in a contact. Only continue with the already created conversation.

Describe the solution you'd like

I can manipulate this via the API, but the frontend lets my agents create new conversations. If I had that possibility, I could block the API channel so that agents don't create new conversations in the same channel for the contact.

Describe alternatives you've considered

A simple option, in channel configuration, with true or false, who enable or disable create new conversations.

Additional context

https://discord.com/channels/647412545203994635/1138794306057027634