[FEATURE] - Check validity

[hayssams]

Description

This ticket lists all validity checks that should be made on the project before executing it.

• Check that connection references are valid and reference existing connections
• Make sure that disposition and merge options are compatible
• Make sure that every expected input files are defined depending on the task launched. Eg: an error is thrown in Attribute (Should never happen: empty list of attributes) but is related to the function call ai.starlake.schema.model.Attribute.sparkType(Attribute.scala:249)) and doesn't help the user to understand what is its mistakes and how to resolve it.

[tiboun]

• command should be checked before loading anything otherwise it takes time just to print command usage.