feat: add 'LoadTocCache' in Download message when starting a preheat job

[SouthWest7]

Description

This commit introduces the LoadToCache field in the Download message, which is used to indicate whether content should be loaded into the memory cache during a preheat job.

• Set LoadToCache to true in three scopes: Single Seed Peer, All Seed Peers and All Peers.

Related Issue

#3742

Motivation and Context

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation Update (if none of the other choices apply)

Checklist

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.