This is the first step for elastic#142753

It's a pretty large and complex PR, which unfortunately is tricky to break down into smaller pieces. Here's a summary of what it contains:

- A new MappingBuilder object that holds Builders for a RootObjectMapper and various MetadataFieldMappers. This handles all merging, meaning that the merge methods on Mapping and Mapper are removed
- MapperService takes input merges as CompressedXContent rather than parsed Mapping objects
- MappingParser returns a MappingBuilder rather than a Mapping
- DocumentParser switches to use Builders to build dynamic mappings
- The various Builder classes for each field type are updated to add a contentType() method, to be used for error messages in merging.
- MetadataFieldMappers now all have Builders
- ObjectMapper merging is all done on the Builder, which is mostly just moving code around but adds some nice simplifications as well.

PR elastic#130540 introduced a bug where indexing a
document with a dotted field name (e.g. "my_vectors.vector1") matched
by a dynamic template for dense_vector or rank_vectors without
explicit dims would fail with:
IllegalStateException: Missing intermediate object.

The bug only manifests when dims are not set in the mapping or
template, triggering the code path that dynamically determines
dimensions from the first indexed document.

The root cause is that when DenseVectorFieldMapper and
RankVectorsFieldMapper dynamically determine dims, they rebuild the
mapper using the document parser's content path. For dynamically
mapped fields with dotted names, the content path includes the field
name itself rather than just the parent object path, producing a
duplicated full path in the rebuilt mapper.

This was fixed on main/9.4 by PR elastic#142754, which refactored dynamic
mapper tracking to use Mapper.Builder objects. This commit provides a
targeted fix for 9.3.x by deriving the correct parent path from the
existing mapper's fullPath instead of relying on the content path.

The same fix is applied to both DenseVectorFieldMapper and
RankVectorsFieldMapper, which shared the same bug.

PR #130540 introduced a bug where indexing a
document with a dotted field name (e.g. "my_vectors.vector1") matched
by a dynamic template for dense_vector or rank_vectors without
explicit dims would fail with:
IllegalStateException: Missing intermediate object.

The bug only manifests when dims are not set in the mapping or
template, triggering the code path that dynamically determines
dimensions from the first indexed document.

The root cause is that when DenseVectorFieldMapper and
RankVectorsFieldMapper dynamically determine dims, they rebuild the
mapper using the document parser's content path. For dynamically
mapped fields with dotted names, the content path includes the field
name itself rather than just the parent object path, producing a
duplicated full path in the rebuilt mapper.

This was fixed on main/9.4 by PR #142754, which refactored dynamic
mapper tracking to use Mapper.Builder objects. This commit provides a
targeted fix for 9.3.x by deriving the correct parent path from the
existing mapper's fullPath instead of relying on the content path.

The same fix is applied to both DenseVectorFieldMapper and
RankVectorsFieldMapper, which shared the same bug.