Support passing api_key (and related params) via kwargs in litellmmodel

[avirajsingh7]

Target Issue : #190

This PR fixes a limitation in litellmmodel where API credentials had to be provided exclusively via environment variables. Even if api_key (or api_base, api_version) was passed during initialization, it was never forwarded to validate_environment or validate_access, causing errors such as:

Required environment variable (keys) from the model are Missing. 
missing_keys: ['OPENAI_API_KEY']

Changes

• Updated validate_environment and validate_access to forward api_key, api_base, and api_version from kwargs.
• This allows per-request credential injection instead of relying only on global environment variables.