CORE-5602 Added metadata template avu validation.

Added pre-hook validation to metadata template avu endpoints.
Includes a fix for the format-avu function when searching for existing AVUs by ID.
Updated donkey.persistence.metadata/find-existing-metadata-template-avu to always look-up an AVU by data ID and owner ID.
Deleting an AVU by ID will validate that AVU exists with the given data ID and owner ID.
CORE-5602 Moved metadata template avu functions.

Moved these service functions into the donkey.services.filesystem.metadata-templates namespace.
CORE-5602 Added metadata template ID validation.

Added metadata template ID validation to /filesystem/:data-id/template-avus endpoints.
CORE-5602 Fixed template avu data ID validation.

Added a valid-uuid-to-path function to donkey.services.filesystem.metadata-templates that will return the path for the given data UUID, or throw a not-found exception.
Used this function in the metadata template avu pre-hook calls since path-exists is not null-safe, and now the valid-uuid-to-path will return an error message referring to the invalid UUID instead of referring to a null path.
CORE-5602 Removed debug logging.