Previous to this commit, only the return labels had a consistent prefix
for their filenames. This meant that all other shipping related
documents had their filenames manually + inconsistently created as
strings in every shipping connector. This lead to difficult to
distinguish documents across different records' chatters.
This commit adds a consistent method for the shipping connectors to use.
Part of task: 3046178 - harmonize shipping label names
ENT PR: odoo/enterprise#43362
Part-of: odoo/odoo#126791
Tiffany Chang (tic)
715a87ce20