ParaView Catalyst blueprint: add support for `params` in execute

Extend the ParaView-Catalyst blueprint as follows:

protocol: 'execute'

params: pass parameters that can change per execute invocation.

• params: (optional) a 'list' of strings.

Next, add a new function in the paraview.catalyst module that Catalyst scripts can use to access these params. Possible names:

• paraview.catalyst.get_params(), or more explicitly
• paraview.catalyst.get_execute_params()

for details on how to implement this, see !4869 (merged).