rust: Refactoring and reduce API surface

Prepare pv & pv_core crates to be released on crates.io:
* Remove any unused API to stay flexible
* Remove utils dependency
* Move cli, tmpfile and version utilities to local utils crate
* Use the new utilities in the pv tools
* Rename Secret into Confidential to avoid confusion of Secret (now
  Confidential) and AddSecret requests.
* Move the uvsecret module out of the request module and change the name
  to secret.
* Cleanup dependencies
* Precise and correct minimal dependency versions
* Inline `Aes256Key::from_digest`

The cleanup ensures that the code also compiles with the dependencies
resolved to their minimal versions using:

$ cargo +nightly -Z minimal-versions update
$ cargo build

For more information refer to this blog post:
https://users.rust-lang.org/t/psa-please-specify-precise-dependency-versions-in-cargo-toml/71277/8

Signed-off-by: Marc Hartmayer <mhartmay@de.ibm.com>
Signed-off-by: Steffen Eiden <seiden@linux.ibm.com>

rust/pv/src/test_utils.rs

@@ -39,12 +39,6 @@ pub fn get_cert_asset_path<P: AsRef<Path>>(path: P) -> PathBuf {
     p
 }
 
-pub fn get_cert_asset_path_string(path: &'static str) -> String {
-    get_cert_asset_path(path)
-        .into_os_string()
-        .into_string()
-        .unwrap()
-}
 /// TEST ONLY! Load an cert
 ///
 /// panic on errors