Add basic state management and file output for Konnect Documents.
Doesn't yet handle writing content to separate files.

Write Document resources' content strings to files within directories
generated from the Service Package Name and Service Version Version.

This does not yet handle escaping "/" characters, which are permitted in
Names and Versions.

Replace the dedicated Service Version and Service Package functions for
Documents with a generic ParentInfoer interface and associated
functions.

A ParentInfoer can indicate an API URL prefix that supports requests for
associated child objects (e.g. the prefix "/api/service_packages"
supports all Document interactions with the "/documents" suffix, as does
"/api/service_versions") and can return the parent's ID and type, (to
provide index keys in the state DB).

Document state schema simplified to use a single foreign/parent object
index. Parents are responsible for generating keys that do not conflict
with other types of parents' keys.

Add tests for document state management.

Add additional document state functions for parity with other state
types.

Add additional error handling to existing document state functions.

- Only create a directory (once) if an SP or SV has an associated
  document.
- Use OS-agnostic path functions.
- Add a utility to escape path separators for entity fields we use as
  directory or file names.

Konnect commands work with additional files when handling documents.
This does not work nicely with stdin/stdout mode, as it'd send some
output (the resource manifest/konnect.yaml) to stdout and some (document
contents) to the filesystem. The simplest option to handle this is to
disable this feature for Konnect commands.