common: custom hf endpoint support

Add support for custom huggingface endpoints via HF_ENDPOINT environment variable

You can now specify a custom huggingface endpoint using the HF_ENDPOINT environment variable when using the --hf-repo flag, which works similarly to huggingface-cli's endpoint configuration.

Example usage:
HF_ENDPOINT=https://hf-mirror.com/ ./bin/llama-cli --hf-repo Qwen/Qwen1.5-0.5B-Chat-GGUF --hf-file qwen1_5-0_5b-chat-q2_k.gguf -p "The meaning to life and the universe is"

The trailing slash in the URL is optional:
HF_ENDPOINT=https://hf-mirror.com ./bin/llama-cli --hf-repo Qwen/Qwen1.5-0.5B-Chat-GGUF --hf-file qwen1_5-0_5b-chat-q2_k.gguf -p "The meaning to life and the universe is"

Author: eternaphia

common/arg.cpp

@@ -656,9 +656,13 @@ static void common_params_handle_model(
                 }
             }
 
-            // TODO: allow custom host
-            model.url = "https://huggingface.co/" + model.hf_repo + "/resolve/main/" + model.hf_file;
-
+            std::string hf_endpoint{"https://huggingface.co/"}; 
+            if(auto const* hf_endpoint_env = getenv("HF_ENDPOINT"); hf_endpoint_env) {
+                hf_endpoint = std::string{hf_endpoint_env};
+                if (hf_endpoint.back() != '/') hf_endpoint += '/';
+            }
+            
+            model.url = hf_endpoint + model.hf_repo + "/resolve/main/" + model.hf_file;
             // make sure model path is present (for caching purposes)
             if (model.path.empty()) {
                 // this is to avoid different repo having same file name, or same file name in different subdirs