Add blob mount support and handle existing blobs

[sudo-bmitch]

Fixes issue

Describe the change

This adds support for the blob mount variant of a blob push. It also adds handling for a blob already existing in the store.

How to verify it

olareg serve --port 5002 --store-type mem
regctl image copy $src localhost:5002/testa
regctl image copy localhost:5002/testa localhost:5002/testb

Changelog text

• Feature: Add support for blob mount.
• Feature: Handle existing blobs in the store.

Please verify and check that the pull request fulfills the following requirements

• Tests have been added or not applicable
• Documentation has been added, updated, or not applicable
• Changes have been rebased to main
• Multiple commits to the same code have been squashed