[AIENG-312] Add a new optimization target option: --similarity

[ono-max]

$ find tests -name "test_*.py" | grep -v tests/data | launchable subset --similarity 0.5 file
tests/test_runners/test_go_test.py
tests/commands/inspect/test_subset.py
tests/test_version.py
tests/test_runners/test_robot.py
...
Your model is currently in training
Launchable created subset 121 for build 221 (test session 296) in workspace launchableinc/mothership

|           |   Candidates |   Estimated duration (%) |   Estimated duration (min) |
|-----------|--------------|--------------------------|----------------------------|
| Subset    |           58 |                   100.00 |                       0.00 |
| Remainder |            0 |                     0.00 |                       0.00 |
|           |              |                          |                            |
| Total     |           58 |                   100.00 |                       0.00 |

Run `launchable inspect subset --subset-id 121` to view full subset details

[kohsuke]

See the server side PR for more substantial discussion. +1 here so as not to block a good work from going live.