add llamastack support

[wukaixingxp]

Description

Add llamastack support to Continue. Most of the functions are just calling the openai compatible endpoints, except the completion endpoint for AutoComplete are customized as openai did not have completion anymore.

Checklist

• [] I've read the contributing guide
• [] The relevant docs, if any, have been updated or created
• [] The relevant tests, if any, have been updated or created

Screenshots

Tested and passing 7 tests, the failing one is due to model problem.