cli: Add "fastpath plan exec" command

Implement the main event; "fastpath plan exec" takes a user-supplied
plan yaml file, which describes the SUT, the configs and the benchmarks
that should be run for each config. It executes the plan and retrieves
all the results, outputting them in a standardised CSV format.
Additionally, it outputs logs from every iteration of every benchmark
for every config.

For now, it doesn't yet actually invoke the benchmark container on the
SUT. Instead it simply generates a mock results.csv file on the SUT,
then copies it back and processes it as if the container had generated
it. Except for this detail, all flows are implemented as they should be.
This stub will be replaced in the near future once we have our first
benchmark container available.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>