[Variant] Support read-only metadata builders (#8208)

# Which issue does this PR close?

- Closes #8152

# Rationale for this change

When manipulating existing variant values (unshredding, removing fields,
etc), the metadata column is already defined and already contains all
necessary field ids. In fact, defining new/different field ids would
require rewriting the bytes of those already-encoded variant values. We
need a way to build variant values that rely on an existing metadata
dictionary.

# What changes are included in this PR?

* `MetadataBuilder` is now a trait, and most methods that work with
metadata builders now take `&mut dyn MetadataBuilder` instead of `&mut
MetadataBuilder`.
* The old `MetadataBuilder` struct is now `BasicMetadataBuilder` that
implements `MetadataBuilder`
* Define a `ReadOnlyMetadataBuilder` that wraps a `VariantMetadata` and
which also implements `MetadataBuilder`
* Update the `try_binary_search_range_by` helper method to be more
general, so we can define an efficient `VariantMetadata::get_entry` that
returns the field id for a given field name.

# Are these changes tested?

Existing tests cover the basic metadata builder. New tests added to
cover the read-only metadata builder.

# Are there any user-facing changes?

The renamed `BasicMetadataBuilder` (breaking), the new `MetadataBuilder`
trait (breaking), and the new `ReadOnlyMetadataBuilder`.

Author: scovich

parquet-variant-compute/src/variant_array_builder.rs

@@ -21,7 +21,7 @@ use crate::VariantArray;
 use arrow::array::{ArrayRef, BinaryViewArray, BinaryViewBuilder, NullBufferBuilder, StructArray};
 use arrow_schema::{ArrowError, DataType, Field, Fields};
 use parquet_variant::{ListBuilder, ObjectBuilder, Variant, VariantBuilderExt};
-use parquet_variant::{MetadataBuilder, ParentState, ValueBuilder};
+use parquet_variant::{ParentState, ValueBuilder, WritableMetadataBuilder};
 use std::sync::Arc;
 
 /// A builder for [`VariantArray`]
@@ -74,7 +74,7 @@ pub struct VariantArrayBuilder {
     /// Nulls
     nulls: NullBufferBuilder,
     /// builder for all the metadata
-    metadata_builder: MetadataBuilder,
+    metadata_builder: WritableMetadataBuilder,
     /// ending offset for each serialized metadata dictionary in the buffer
     metadata_offsets: Vec<usize>,
     /// builder for values
@@ -96,7 +96,7 @@ impl VariantArrayBuilder {
 
         Self {
             nulls: NullBufferBuilder::new(row_capacity),
-            metadata_builder: MetadataBuilder::default(),
+            metadata_builder: WritableMetadataBuilder::default(),
             metadata_offsets: Vec::with_capacity(row_capacity),
             value_builder: ValueBuilder::new(),
             value_offsets: Vec::with_capacity(row_capacity),