Add `/project/...` route to validate a path exists in current project

[mferrera]

As mentioned in #99 (comment) it will be useful to ensure a path provided by a user exists within a the current project directory. We could consider exposing this as an endpoint which returns the relative path so that the GUI can provide a correct relative path to other endpoints.

It may also be useful to distinguish if the path points to something that exists, or is simply within the project (whether or not it exists).

To be refined, in particular what this route should be named.

[mferrera]

This is currently "wontfix", i.e. do not implement, as it's not a problem we have yet, but it may be useful later.