feat: Add example for how to write unit-testable rust canisters (#1270)

This provides an example of best practices for structuring canisters for easier unit testing, as well as an example of a PocketIC test that automatically updates the wasm under test when running the test.  It contains multiple examples for how to write good tests in canisters that are easy to maintain.

Author: max-dfinity

.github/CODEOWNERS

@@ -83,6 +83,7 @@
 /rust/tokenmania/ @dfinity/ninja-devs
 /rust/token_transfer/ @dfinity/growth
 /rust/token_transfer_from/ @dfinity/growth
+/rust/unit_testable_rust_canister @dfinity/governance-team
 /rust/vetkd/ @dfinity/crypto-team
 /rust/who_am_i/ @dfinity/ninja-devs
 /rust/x509/ @dfinity/crypto-team