Allow overriding constrains

[pablochacin]

The documentation states the following

Version constraints can be specified in several sources (pragma, environment, manifest) but cannot be overwritten. That is, for a given extension, the version constraints from different sources must either be equal, or only one source can contain a version constraint.

In some scenarios, it can be convenient to provide a default version for extensions in the form of a manifest but allow users to override the version of a specific extension in the script. For example, when running tests in the cloud, the user will not have control of the manifest, so using the in-script pragmas will be the only way to change the defaults.