Cargo.toml: add vendor-filter metadata

[bgurney-rh]

Related: stratis-storage/project#642
Related: stratis-storage/project#265

[bgurney-rh]

Amending all-features = true to ensure that pretty-hex is included

[mulkieran]

The * configuration works, but I encountered an error:

Gathering metadata for platforms
error: Executing cargo metadata: Error during execution of `cargo metadata`: error: failed to run `rustc` to learn about target-specific information

Caused by:
  process didn't exit successfully: `rustc - --crate-name ___ --print=file-names --target m68k-unknown-linux-gnu --crate-type bin --crate-type rlib --crate-type dylib --crate-type cdylib --crate-type staticlib --crate-type proc-macro --print=sysroot --print=split-debuginfo --print=crate-name --print=cfg` (exit status: 1)
  --- stderr
  error: could not create LLVM TargetMachine for triple: m68k-unknown-linux-gnu: No available targets are compatible with triple "m68k-unknown-linux-gnu"

The associated issue is: coreos/cargo-vendor-filterer#50 .

We could consider filtering by tier.

[mulkieran]

Add tier = "2" so this will build with Fedora Rust release, and we can consider this ready to merge.

[bgurney-rh]

Amended tier = "2"