Add custom_host support to allow proxies

[VisargD]

Problem statement:

Gateway currently does not allow hitting LLM which are under some other proxy because the base url for each provider is static in code. Many organisations or users have openai, anthropic, cohere, etc access under a proxy server. So instead of using api.openai.com, their request should be able to hit their custom proxy url to use that proxy. One of the main reasons of such proxy is to have a custom auth, spend limit, rate limit, etc. which organisations assign to their members instead of exposing the actual LLM key.

Related issue: #124

Solution:

• add a custom_host setting in configs where users can pass their proxy url and gateway will just add the endpoint to it.
• add a x-portkey-custom-host header which does the same as config setting but it will be applied globally and will overwrite custom_host passed in config.