feat: Introduce custom-host provider with configurable base URL and API key support.

[Jaiaggarwaaaaal]

feat: Introduce custom-host provider for arbitrary proxy support

Description:
This PR introduces a new custom-host provider to the Gateway. This feature allows users to proxy requests to any configurable base URL, enabling integration with LLM providers or services that are not natively supported by the Gateway. It supports dynamic configuration of the target endpoint and API key authentication.

Detailed changes:

• New Provider: Added custom-host provider implementation in src/providers/custom-host/.
• Registration: Registered the new provider in src/providers/index.ts.
• Type Definitions: Updated src/types/requestBody.ts to include custom-host configuration types.
• Global Config: Updated src/globals.ts to support the new provider constant.

Tests Run/Test cases added:

• Verified request forwarding to a custom HTTP endpoint.
• Verified correct propagation of Authorization headers/API keys.
• Tested error handling for unreachable hosts.

Type of Change:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)

curl --location 'http://localhost:8788/v1/v2/vectordb/collections/list' \
--header 'x-portkey-custom-host: https://httpbin.org/anything' \
--header 'Authorization: Bearer test' \
--header 'x-portkey-provider: openai' \
--header 'Content-Type: application/json' \
--data '{"test": "payload"}'

curl --location 'http://localhost:8788/v1/chat/completions' \
--header 'x-portkey-custom-host: https://httpbin.org/anything' \
--header 'Authorization: Bearer test' \
--header 'x-portkey-provider: openai' \
--header 'Content-Type: application/json' \
--data '{"model": "gpt-4o", "messages": [{"role": "user", "content": "hi"}]}'

curl --location 'http://localhost:8788/v1/upload' \
--header 'x-portkey-custom-host: https://httpbin.org/anything' \
--header 'x-portkey-provider: openai' \
-F 'file=@/Users/jaiaggarwal/gateway/package.json' \
-F 'purpose=fine-tune'

[narengogi]

is this a test file?
you might've commited it by mistake

[narengogi]

i see that there is no request body in your requests

can you test the behaviour with one of the unified routes like /v1/chat/completions and any arbitrary route like v1/my/custom/route ?

and can you try with different payloads like json and multipart-formdata?