Make base server endpoint configurable

[sjmonson]

Is your feature request related to a problem? Please describe.
Sometimes OpenAI-compatible endpoints are hosted on differing paths from /v1/... (e.g. /v3/..., /v1beta/...)

Describe the solution you'd like
Support a config option for configuring the base endpoint. Something like GUIDELLM__OPENAI__BASE_ENDPOINT=/v1.