feat: support custom HTTP headers for model providers (#1473)

This adds support for two new model provider config options:

- `http_headers` for hardcoded (key, value) pairs
- `env_http_headers` for headers whose values should be read from
environment variables

This also updates the built-in `openai` provider to use this feature to
set the following headers:

- `originator` => `codex_cli_rs`
- `version` => [CLI version]
- `OpenAI-Organization` => `OPENAI_ORGANIZATION` env var
- `OpenAI-Project` => `OPENAI_PROJECT` env var

for consistency with the TypeScript implementation:


bd5a9e8ba9/codex-cli/src/utils/agent/agent-loop.ts (L321-L329)

While here, this also consolidates some logic that was duplicated across
`client.rs` and `chat_completions.rs` by introducing
`ModelProviderInfo.create_request_builder()`.

Resolves https://github.com/openai/codex/discussions/1152

codex-rs/config.md

@@ -76,6 +76,22 @@ env_key = "AZURE_OPENAI_API_KEY"  # Or "OPENAI_API_KEY", whichever you use.
 query_params = { api-version = "2025-04-01-preview" }
 ```
 
+It is also possible to configure a provider to include extra HTTP headers with a request. These can be hardcoded values (`http_headers`) or values read from environment variables (`env_http_headers`):
+
+```toml
+[model_providers.example]
+# name, base_url, ...
+
+# This will add the HTTP header `X-Example-Header` with value `example-value`
+# to each request to the model provider.
+http_headers = { "X-Example-Header" = "example-value" }
+
+# This will add the HTTP header `X-Example-Features` with the value of the
+# `EXAMPLE_FEATURES` environment variable to each request to the model provider
+# _if_ the environment variable is set and its value is non-empty.
+env_http_headers = { "X-Example-Features": "EXAMPLE_FEATURES" }
+```
+
 ## model_provider
 
 Identifies which provider to use from the `model_providers` map. Defaults to `"openai"`.