Expose `SystemMeta` for system exploration and configuration

## What problem does this solve or what need does it fill?

Currently, there is no unified way to access `SystemMeta`.
We should define a set of methods in `System`, which would work for all: function, exclusive and compound systems.
This is especially needed for system param warning, which currently can be configured only for function systems.

## What solution would you like?

Introduce `System::system_metas(&self)` and `System::system_metas_mut(&mut self)` which would return iterator/vector/slice that we can work with.
@alice-i-cecile suggested a vector as a good starting point.

## What alternative(s) have you considered?

Reflection or separate structure for storing system configuration.