Secret memory with memfd_secret (#321)

Implements:
- An additional allocator to use memfd_secret(2) and guard pages using mmap(2), implemented in quininer/memsec#16
- An allocator that abstracts away underlying allocators, and uses specified allocator set by rosenpass_secret_memory::policy functions (or a function that sets rosenpass_secret_memory::alloc::ALLOC_INIT
- Updates to tests- integration, fuzz, bench: some tests use procspawn to spawn multiple processes with different allocator policies

rosenpass/Cargo.toml

@@ -49,6 +49,7 @@ criterion = { workspace = true }
 test_bin = { workspace = true }
 stacker = { workspace = true }
 serial_test = {workspace = true}
+procspawn = {workspace = true}
 
 [features]
 enable_broker_api = ["rosenpass-wireguard-broker/enable_broker_api"]