PR-6293 Add storage for querying CMR

[reweeden]

Adds a new storage type for making HTTP requests via requests. There is also a convenience class with a few special parameters that are intended to make passing the CMR configuration a bit nicer.

In order to make this actually usable we also needed to add some way of injecting dynamic values into the objects returned from the source configuration. We implement that internally with a new class called ContextValue which is a placeholder containing a jsonpath where a value should be pulled from the Context object.

The syntax we use for defining a context value in the source config is a $ at the start of a string. This can be escaped like this $$ in which case the string will be interpreted as a literal string starting with a single $. For example $.meta.foo will become a ContextValue(path="$.meta.foo"), however $$.meta.foo will become the string $.meta.foo.