Add a dynamic asset to pick up file changes

[jlubken]

The Asset class is used to load text files from a directory tree.

By design this is done once as part of yaml deserialization when the configuration is first loaded to separate early configuration failures from late runtime failures: dev-ops principle to shift all error to the left.

This is problematic for working on sql queries in a notebook since components must be deserialized again after every sql file change.

Create a development version of Asset registered as !asset for notebook use that dynamically loads content from the filesystem.