feat: add 'load_to_cache' field to Download message

[SouthWest7]

Description

load_to_cache indicates whether the content downloaded will be stored in the cache storage.

• Cache storage: When a user triggers a preheat task request, and if the peer has the cache storage feature enabled, the downloaded piece content will be stored in the cache. Subsequently, when other peers request downloading the same content, the preheated peer can upload the piece content directly from memory, bypassing disk I/O and reducing the overhead associated with disk reads.
• If the field is set to true, it signifies that the task is part of a preheat process and the content downloaded will be stored in the cache, assuming the feature is enabled.

Related Issue

dragonflyoss/dragonfly#3742

Motivation and Context

[gaius-qi]

Add bool load_to_cache = 21; to Download message.

[gaius-qi]

Use cache storage. Rewrite of description, because I don't know what you are trying to describe.

[gaius-qi]

LGTM