Add assets folder

[IgorShcherbakov]

At the moment, when deploying a project, several subfolders are created (for example, checkpoints or expectations) and this is very convenient to manage. The question is: why can’t we do the same with assets? Add an asset folder, and refer to the names in great_expectations.yml, just like checkpoint.yml refers to an asset. Currently all requests are stored in great_expectations.yml, and with the addition of new checks, the file becomes very difficult to maintain. Adding modularity to assets would improve usability of the product.