enhance: add configurable storage writer format (#50067)

issue: #50147

This change pulls the Vortex writer-format switch into its own focused
update. The goal is to make the insert data storage format configurable
from Milvus config, so we can turn Vortex on for CI validation without
mixing it with the unrelated AssertInfo cleanup from the previous PR.

The implementation adds dataNode.storage.format and wires it through
paramtable as DataNodeCfg.StorageFormat, with vortex as the current
default for this branch. When storagev2 builds milvus-storage
properties, it now passes the selected value through writer.format,
which matches the updated milvus-storage property contract.

This also updates the C++ writer path to use PROPERTY_WRITER_FORMAT
instead of the old PROPERTY_FORMAT key. That keeps the growing-segment
flush path aligned with the new storage property naming while still
forcing the TEXT flush writer to parquet where that path expects parquet
output.

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Signed-off-by: jiaqizho <jiaqi.zhou@zilliz.com>