Adds infrastructure so `gradle :docs:check` will extract tests from
snippets in the documentation and execute the tests. This is included
in `gradle check` so it should happen on CI and during a normal build.

By default each `// AUTOSENSE` snippet creates a unique REST test. These
tests are executed in a random order and the cluster is wiped between
each one. If multiple snippets chain together into a test you can annotate
all snippets after the first with `// TEST[continued]` to have the
generated tests for both snippets joined.

Snippets marked as `// TESTRESPONSE` are checked against the response
of the last action.

See docs/README.asciidoc for lots more.

Closes elastic#12583. That issue is about catching bugs in the docs during build.
This catches *some* bugs in the docs during build which is a good start.