This commit adds support to enable bulk upserts to use an index's
default pipeline. Bulk upsert, doc_as_upsert, and script_as_upsert
are all supported.

However, bulk script_as_upsert has slightly surprising behavior since
the pipeline is executed _before_ the script is evaluated. This means
that the pipeline only has access the data found in the upsert field
of the script_as_upsert. The non-bulk script_as_upsert (existing behavior)
runs the pipeline _after_ the script is executed. This commit
does _not_ attempt to consolidate the bulk and non-bulk behavior for
script_as_upsert.

This commit also adds additional testing for the non-bulk behavior,
which remains unchanged with this commit.

fixes elastic#36219