Allow user input for querying a metadata source

[jsheunis]

This functionality would be an extension of the piecewise querying specified here: #68.

shacl-vue currently supports (via the InstancesSelectEditor) the selection of a specific named node from a list of nodes that have the same rdf:type. See e.g. this screenshot:

When the user selects the dropdown, the list is populated dynamically by querying the (currently browser-local) graph store for more named nodes that have the same type as the range of the slot. When the shift is made to querying some arbitrary endpoint that could potentially be the source of thousands/millions of such nodes (think e.g. "files", or "samples"), this action will require a very noticeable duration of time which will be bad for UX and general scalability. Therefore, it would be useful if the user interface for such a step could include a way to constrain the query, such as a text field for searching for a specific node (e.g. a specific file name) amongst a multitude of possibilities.

More concretely, an idea could be to update the InstancesSelectEditor to include a text field as user input and that the "dynamically populated list" would rather be replaced by the result of the query for all triple subjects that are of the specified type, and also have triple object values that are a partial match for the entered text.