c-s CLI: columns and population options

[muzarski]

Changes

• Refactored the Param trait, by extracting most of the common logic to the wrapping type
• introduced DistributionParam
• implemented parsing logic for -col option
• implemented parsing logic for -pop option

[muzarski]

v2:

• rebased on top of master
• removed utility functions decomposing the CLI argument into distribution description payload - using functionalities implemented in src/distribution.rs instead
• removed DistributionParser trait (with its implementations)
• moved specific distribution parsing logic from DistributionParser implementations to specific DistributionFactories.
• moved description printing from DistributionParser implementations to distribution factories
• changed the type returned from DistributionFactory::create from Arc<dyn Distribution> to Box<dyn Distribution>. Some of the distributions are not shared (Box should be used), when at the same time there is a distribution that will be shared across multiple tokio tasks (Arc). It's much easier to convert Box<dyn Object> to Arc<dyn Object> (.into()), than the other way around.
• added two new commits (since runtime logic has already been merged):
  • using user-provided distribution (-pop dist parameter) as distribution which RowGenerator samples pk_seeds from
  • using -col option parameters in the runtime, including:
  • adjusted table creation/insertion queries based on the columns provided by the user (-col n or -col names parameters)
  • using cell-size distribution (RowGenerator samples the size of the blob data that is going to be generated from this distribution) based on user-provided distribution (-col size parameter)

[muzarski]

v3:

• removed unnecessary TODO next to RowGeneratorFactory::settings field.

[muzarski]

v4:

• rebased on top of master
• fixed review comments, including most important:
  • removed DistributionParam, using SimpleParam<Box<dyn DistributionFactory>> instead
  • implemented Parsable for Box<dyn DistributionFactory> (moved parsing logic from DistributionParam)
  • extended SimpleParam struct by additional_desc field

[muzarski]

v5: fixed review comments, ready to merge