Remove dependence upon `pki-playground` and `attest-mock` binaries

[andrewjstone]

When running tests for sprockets-tls, a build.rs file is run that requires pki-playground and attest-mock binaries to be installed. With oxidecomputer/pki-playground#126 and the implementation of
oxidecomputer/dice-util#303 we will be able to instead generate the key/cert/log/corpus files from test code using dev-dependencies on these libraries. This will remove the need for the build.rs for these tests.

It looks like the xtask to run the examples also relies upon the build.rs to generate these files. We can instead have the xtask generate the files instead via code.

Doing this will make it easier for developers to get started with sprockets. They can just clone the repo and make test.