[Variant] Variant builder improvements

[scovich]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

This is a tracking ticket for work items identified by

• [Variant] [Pathfinding] Variant builder rollback, variant array builder, and read-only metadata builder #8167

Describe the solution you'd like

Several things:

• [Variant] ObjectBuilder does not reliably check for duplicates #8170
• [Variant] ObjectBuilder should detect duplicates at insertion time, not at finish #8180
• [Variant] Refactor ParentState to track and rollback state on behalf of its owning builder #8182
• Make ObjectBuilder::finish signature infallible #8184
• Rework ValueBuilder API to work with ParentState for reliable nested rollbacks #8188
• [Variant] Rename ValueBuffer as ValueBuilder #8186
• Make ValueBuilder, ParentState, etc public
• VariantArrayBuilder tracks starting offsets instead of (offset, len) pairs #8192
• VariantArrayBuilder uses ParentState for simpler rollbacks #8205
• Add fallible and infallible pairs for new_object and new_list to VariantBuilderExt trait.
• Make metadata builders reusable, with finish taking &mut self instead of self
• Turn MetadataBuilder into a trait, with the current metadata builder as the canonical implementation
• [Variant] Support creating Variants with pre-existing Metadata #8152
• Support appending raw bytes to variant objects and lists #8217
• [Variant] Split builder.rs into several smaller files #8218

Describe alternatives you've considered

• [Variant] VariantBuilder with VariantMetadata instead of MetadataBuilder #7915

Additional context

Part of the variant shredding saga.

[sdf-jkl]

@scovich @alamb Can we close this?

[alamb]

Good call -- done!