[proxy] Add SCRAM auth mechanism implementation (#1050)

* [proxy] Add SCRAM auth

* [proxy] Implement some tests for SCRAM

* Refactoring + test fixes

* Hide SCRAM mechanism behind `#[cfg(test)]`

Currently we only use it in tests, so we hide all relevant
module behind `#[cfg(test)]` to prevent "unused item" warnings.

proxy/src/parse.rs

@@ -0,0 +1,18 @@
+//! Small parsing helpers.
+
+use std::convert::TryInto;
+use std::ffi::CStr;
+
+pub fn split_cstr(bytes: &[u8]) -> Option<(&CStr, &[u8])> {
+    let pos = bytes.iter().position(|&x| x == 0)?;
+    let (cstr, other) = bytes.split_at(pos + 1);
+    // SAFETY: we've already checked that there's a terminator
+    Some((unsafe { CStr::from_bytes_with_nul_unchecked(cstr) }, other))
+}
+
+pub fn split_at_const<const N: usize>(bytes: &[u8]) -> Option<(&[u8; N], &[u8])> {
+    (bytes.len() >= N).then(|| {
+        let (head, tail) = bytes.split_at(N);
+        (head.try_into().unwrap(), tail)
+    })
+}