feat: Add blackhole storage, for benchmarking

[yurishkuro]

Which problem is this PR solving?

• When benchmarking jaeger-v2 vs. jaeger-collector, it's useful to have a no-op storage.

Description of the changes

• Add blackhole storage type.

How was this change tested?

• unit tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files	Coverage Δ
plugin/storage/blackhole/blackhole.go	100.00% <100.00%> (ø)
plugin/storage/blackhole/factory.go	100.00% <100.00%> (ø)
plugin/storage/factory.go	98.41% <100.00%> (+0.01%)	⬆️
plugin/storage/factory_config.go	100.00% <ø> (ø)

... and 2 files with indirect coverage changes

📢 Thoughts on this report? Let us know!.

[albertteoh]

blackhole storage is a cool name, although "no-op" storage is immediately intuitive to me, probably because it's a common parlance.

[yurishkuro]

I was going for cool.