refactor: enhance forward declarations in type_fwd.h to reduce compile dependencies

[HeartLinked]

Enhanced the forward declaration system in type_fwd.h to reduce compilation dependencies and improve build times.

Changes Made

• Added missing forward declarations for commonly used classes
• Reorganized declarations into logical groups with descriptive comments
• Removed outdated TODO comment regarding (These classes were already properly declared):
  • StructLike
  • MetadataUpdate
  • UpdateRequirement
  • AppendFiles

[zhjwpku]

Enhanced the forward declaration system in type_fwd.h to reduce compilation dependencies and improve build times.

Changes Made

• Added missing forward declarations for commonly used classes
• Reorganized declarations into logical groups with descriptive comments
• Removed outdated TODO comment regarding (These classes were already properly declared):

I don't think this is true, so I'd prefer to leave the TODO comment as is.

• StructLike
• MetadataUpdate
• UpdateRequirement
• AppendFiles

Other than that, the PR looks good.

[Fokko]

Thanks for working on this @HeartLinked, and thanks for the review @zhjwpku & @wgtmac 🙌