feat(rust): add GetInfo/GetObjects/GetStatistics result builders

[fornwall]

What

Adds public GetInfoBuilder, GetObjectsBuilder, and GetStatisticsBuilder to adbc_core::schemas::builder with typed append_* / finish() -> RecordBatch APIs that own the union type-ids, list offsets, and null handling for the nested get_info / get_objects / get_statistics result shapes.

Previously drivers hand-rolled the nested union/list/struct assembly inline. Go's driverbase and the C++ driver framework already absorb exactly this for their drivers.

Note: this branch was rebased after the DataFusion driver was removed upstream (apache#4455). The original version also rewrote that driver; this PR now only refactors the dummy driver. The builders remain useful for the dummy driver and any future Rust driver.

Changes

• New module rust/core/src/schemas/builder.rs:
  • GetInfoBuilder — append_string / append_bool / append_int64 / append_int32_bitmask / append_string_list / append_int32_to_int32_list_map (all six union variants).
  • GetObjectsBuilder — hierarchical append_catalog → append_db_schema → append_table → append_column / append_constraint, with helper structs ColumnSchema, TableConstraint, ConstraintUsage. Respects ObjectDepth, emitting null lists for levels below the requested depth.
  • GetStatisticsBuilder — append_catalog → append_db_schema → append_statistic, with a StatisticValue enum for the value union.
  • Field definitions are pulled directly from the schemas::* constants, so array and schema types cannot drift apart.
• Rewrote the dummy driver to use the builders (get_info, get_objects, get_statistics), producing byte-identical output. Removed the now-unused arrow-buffer dependency; added it to adbc_core.

Testing

• cargo test -p adbc_core -p adbc_dummy — all pass, including the dummy driver's exported-vs-native FFI round-trip equality tests (confirming output is unchanged) and 4 new unit tests + a doctest in the builder module.
• cargo build --workspace — clean.

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