Add footer

- Add a footer to each mail resulting from a split into an encrypted and
  an unencrypted version. The footer contains the recipients of the mail
  which are excluded in the corresponding stream.
- Move the construction of the `HuskContext` to the `mail`
  milter-command, as `helo` is used by scrapers to detect mailservers.
- Use `CertD::trust_root()` to get the trust root - it creates one if no
  root can be found.
- Add `chrono` dependency.

Cargo.toml

@@ -14,6 +14,7 @@ rust-version = "1.85"
 aho-corasick = "1.1.4"
 anyhow = "1.0.100"
 bytes = "1.11.1"
+chrono = "0.4.45"
 clap = { version = "4", features = ["derive"] }
 fastrand = "2.4.1"
 indymilter = "0.3.0"
@@ -46,3 +47,6 @@ clap_complete = "4"
 [profile.release]
 lto = true
 codegen-units = 1
+
+[patch.crates-io]
+sequoia-cert-store = { git = "https://gitlab.com/sequoia-pgp/sequoia-cert-store", branch = "malte/assert_base_dir" }