benchmarking/block: Add block benchmarking example

[Willmish]

The block benchmarking system got stale and has been sitting on a branch (https://github.com/au-ts/sddf/tree/szymon/benchmark_blk_rebased) waiting for the sdmmc driver PR (#273) to get merged in.

Opening this to track progress for myself, outstanding tasks:

• Bring up to speed so it runs on most recent releases of microkit and main branch of sddf on qemu (aarch64, maybe riscv64 too if PMU support is enough)
• instead of waiting for the rust sdmmc driver getting merged in, make it runnable using the blk driver for maaxboard
• Figure out SD Host controller power cycling interface (PPC to the driver? separate command? which client gets to do it? maybe separate PR)
• Add script for benchmarking load generation (currently uses FIO traces from Linux runs of the benchmark) which get embedded into the executable. Parameters needed: partition size (or just size <partition size), which benchmarks are enabled (random read, random write, sequential read, sequential write), number of requests per benchmark, number of reruns. Alternatively: use FIO to generate the trace as a dry run?
• Add documentation for benchmarking and reproducing same benchmarks on Linux