Support adding root cert via env CARGO_HTTP_CAINFO, SSL_CERT_{FILE, PATH} (#774)

* Support for custom root cert in `binstalk_downloader::remote::Client`
* Support adding root cert via env `CARGO_HTTP_CAINFO`, `SSL_CERT_{FILE, PATH}`

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

crates/binstalk-downloader/src/remote/certificate.rs

@@ -0,0 +1,67 @@
+use std::{env, ffi::OsStr, fs, io, path::Path};
+
+use compact_str::CompactString;
+use reqwest::tls;
+use thiserror::Error as ThisError;
+
+use super::ReqwestError;
+
+#[derive(Debug, ThisError)]
+pub enum OpenCertificateError {
+    #[error(transparent)]
+    Reqwest(#[from] ReqwestError),
+
+    #[error(transparent)]
+    Io(#[from] io::Error),
+
+    #[error("Expected extension .pem or .der, but found {0:#?}")]
+    UnknownExtensions(Option<CompactString>),
+}
+
+#[derive(Clone, Debug)]
+pub struct Certificate(pub(super) tls::Certificate);
+
+impl Certificate {
+    /// Open Certificate with path specified by the environment variable `name`
+    pub fn from_env(name: impl AsRef<OsStr>) -> Result<Option<Self>, OpenCertificateError> {
+        Self::from_env_inner(name.as_ref())
+    }
+
+    fn from_env_inner(name: &OsStr) -> Result<Option<Self>, OpenCertificateError> {
+        env::var_os(name)
+            .map(|value| Self::open_inner(Path::new(&value)))
+            .transpose()
+    }
+
+    /// Open Certificate on disk and automatically detect its format based on
+    /// its extension.
+    pub fn open(path: impl AsRef<Path>) -> Result<Self, OpenCertificateError> {
+        Self::open_inner(path.as_ref())
+    }
+
+    fn open_inner(path: &Path) -> Result<Self, OpenCertificateError> {
+        let ext = path.extension();
+
+        let f = if ext == Some(OsStr::new("pem")) {
+            Self::from_pem
+        } else if ext == Some(OsStr::new("der")) {
+            Self::from_der
+        } else {
+            return Err(OpenCertificateError::UnknownExtensions(
+                ext.map(|os_str| os_str.to_string_lossy().into()),
+            ));
+        };
+
+        Ok(f(fs::read(path)?)?)
+    }
+
+    /// Create a Certificate from a binary DER encoded certificate
+    pub fn from_der(der: impl AsRef<[u8]>) -> Result<Self, ReqwestError> {
+        tls::Certificate::from_der(der.as_ref()).map(Self)
+    }
+
+    /// Create a Certificate from a PEM encoded certificate
+    pub fn from_pem(pem: impl AsRef<[u8]>) -> Result<Self, ReqwestError> {
+        tls::Certificate::from_pem(pem.as_ref()).map(Self)
+    }
+}