pubsys: improve stability of SSM parameter validation

[cbgbt]

Issue number:

Closes #347
Closes #346

Description of changes:

    workspace: homogenize dependencies
    
    This changes helps prepare the workspace to use workspace dependencies
    by using the same version requirements and feature sets for common
    dependencies of crates in the workspace.

    workspace: move to workspace dependencies

    pubsys: implement Display for ValidateSsm error

    pubsys: rate-limit SSM GetParameter calls

    pubsys: retry SSM validation on any failure

Testing done:

• published and validated AMIs with SSM parameters

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[bcressey]

What does lib = true do here? It's sort of surprising since these are all binaries.

Can we move the artifact dependency stuff to twoliter/Cargo.toml and leave just the version and path here? It seems a little weird for cases where we have a cross crate dependency (like pubsys depends on buildsys).

[cbgbt]

What does lib = true do here? It's sort of surprising since these are all binaries.

buildsys depends on pipesys as a library, and you need to add lib = true if you use artifact dependencies to tell Cargo to build both.

   Compiling buildsys v0.1.0 (/home/ec2-user/twoliter/tools/buildsys)
error[E0433]: failed to resolve: use of undeclared crate or module `pipesys`
  --> tools/buildsys/src/builder.rs:20:5
   |
20 | use pipesys::server::Server as PipesysServer;
   |     ^^^^^^^ use of undeclared crate or module `pipesys`

---

Can we move the artifact dependency stuff to twoliter/Cargo.toml and leave just the version and path here?

I really wanted to do it this way, but Cargo wouldn't cooperate. e.g. if you move the artifact declaration for pubsys to twoliter/Cargo.toml, cargo build says:

cargo clippy --locked -- -D warnings --no-deps
warning: /home/ec2-user/twoliter/twoliter/Cargo.toml: unused manifest key: dependencies.pubsys.artifact
warning: twoliter v0.4.4 (/home/ec2-user/twoliter/twoliter) ignoring invalid dependency `pubsys` which is missing a lib target

... and then eventually

    Checking pubsys v0.1.0 (/home/ec2-user/twoliter/tools/pubsys)
error: environment variable `CARGO_BIN_FILE_PUBSYS` not defined at compile time
  --> twoliter/src/tools.rs:17:38
   |
17 | const PUBSYS: &[u8] = include_bytes!(env!("CARGO_BIN_FILE_PUBSYS"));
   |                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: Cargo sets build script variables at run time. Use `std::env::var("CARGO_BIN_FILE_PUBSYS")` instead
   = note: this error originates in the macro `env` (in Nightly builds, run with -Z macro-backtrace for more info)

error: could not compile `twoliter` (bin "twoliter") due to 1 previous error
make: *** [Makefile:14: clippy] Error 101

[bcressey]

nit:

Suggested change

	/// Returns when the rate limit is satisifed.
	/// Returns when the rate limit is satisfied.