Tool for local SDK search index benchmark

[isoos]

• SDK search index improvements (tracking issue) #8670
• This tool is similar to https://github.com/dart-lang/pub-dev/blob/master/app/bin/tools/search_benchmark.dart only for SDK indexes, using the Flutter index by default, as it is larger.
• The local profiling showed that most time is spent in the top-k (top-3) result selection. I didn't see any trivial changes there that improved the outcome.
• The overall time spent in the SDK search method is less than 2 ms on my localhost - it is likely not a big contributor to search latencies (which were in the 30-40 ms range last time I've checked). Still, it may be worth to look into top-k selection, as the regular search index also uses it when sorting/selecting the results.