Ability to run multi assert steps in parallel

[Suremaker]

Description

Currently it is possible to mark scenario or composite step with [MultiAssert] attribute that instructs runner to execute all sub-steps regardless of their status and collect the outcome at the end of execution.

This change should allow to flag those sub-steps to run in parallel, assuming that they are independent.
The use case scenario would be a composite step containing sub-steps describing:

• a combination of input and expected output values for an algortihm,
• a list of (longer runnng) actions that can be executed against existing setup.

A spike is required to determine:

• if flag would be implemented as property of MultiAssertAttribute or separate attribute,
• if it makes sense to apply it to scenarios and composite steps or only composites

Progress

• Feature is implemented,
• Ensured backward-compatibility,
• Ensured good debugging experience