tmp/fmt/dbus: add encoding/decoding layers

Add encoding and decoding layers for the D-Bus formatting module.

This uses the new:

- D-Bus type-system abstraction available in `tmp::fmt::dbus`
- I/O abstractions for streaming data and memory mapped data in
  `tmp::io`.
- Streaming JSON implementation in `tmp::fmt::json`

This is modeled after `c-dvar`, the C implementation of D-Bus Variants
as provided by dbus-broker.

Signed-off-by: David Rheinsberg <david@readahead.eu>

Author: dvdhrm

lib/tmp/Cargo.toml

@@ -19,6 +19,9 @@ readme.workspace = true
 repository.workspace = true
 rust-version.workspace = true
 
+[features]
+polonius = []
+
 [dependencies]
 osi = { workspace = true }
 sys = { workspace = true }