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/Cargo.lock

@@ -414,7 +414,6 @@ name = "pv"
 version = "1.0.0"
 dependencies = [
  "byteorder",
- "clap",
  "curl",
  "log",
  "openssl",
@@ -423,7 +422,6 @@ dependencies = [
  "serde",
  "serde_test",
  "thiserror",
- "utils",
  "zerocopy",
 ]
 
@@ -438,7 +436,6 @@ dependencies = [
  "serde",
  "serde_test",
  "thiserror",
- "utils",
  "zerocopy",
 ]
 
@@ -449,7 +446,6 @@ dependencies = [
  "clap",
  "lazy_static",
  "openssl",
- "openssl-sys",
  "pv_core",
  "rand",
  "regex",
@@ -687,6 +683,12 @@ checksum = "711b9620af191e0cdc7468a8d14e709c3dcdb115b36f838e601583af800a370a"
 [[package]]
 name = "utils"
 version = "0.1.0"
+dependencies = [
+ "clap",
+ "libc",
+ "log",
+ "pv",
+]
 
 [[package]]
 name = "vcpkg"