ai: add models webhook url param

[gioelecerati]

What does this pull request do? Explain your changes. (required)

In order to avoid restarting orchestrators whenever we change the runners list we'd like to have a dynamic way of setting up the -aiModels. Similarly to [-orchWebhookUrl](https://github.com/livepeer/go-livepeer/blob/master/doc/orchwebhook.md?plain=1#L4) that can be used for providing dynamic list or orchestrators on the broadcaster side.

Specific updates (required)

• Add aiModelsWebhookUrl param, accepting a string which can be either a webhook url or a file path
• Webhook gets called every 1 minute, updating the model configs
• Webhook.GetConfigs() returns the most recent config
• Added tests for both url and file updates

How did you test each of these updates (required)

unit tests

Does this pull request close any open issues?

Checklist:

• [X] Read the contribution guide
• [ ] make runs successfully
• [ ] All tests in ./test.sh pass
• [ ] README and other documentation updated
• [ ] Pending changelog updated