[Variant] refactor: Split builder.rs into several smaller files (#8635)

# Which issue does this PR close?
- Closes #8218

# Rationale for this change

# What changes are included in this PR?
Split the `builder.rs` to three files, `/builder/list`,
`/builder/object` and `builder/metadata`

# Are these changes tested?
Yes
# Are there any user-facing changes?
No

Author: Weijun-H