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/pvsecret/src/cli.rs

@@ -3,8 +3,7 @@
 // Copyright IBM Corp. 2023
 
 use clap::{ArgGroup, Args, CommandFactory, Parser, Subcommand, ValueEnum, ValueHint};
-use pv::misc::CertificateOptions;
-use pv::misc::STDOUT;
+use utils::{CertificateOptions, STDOUT};
 
 /// Manage secrets for IBM Secure Execution guests.
 ///