Typesense sink hardcodes action=emplace, so omitted fields persist in the index

[osaton]

The Typesense sink always writes with action=emplace, which merges into an
existing document. Any key not present in the payload keeps its old value, so a
field that disappears from the source row stays in the index and keeps matching
in search.

Stored document:

{ "id": "1", "name": "Shoe", "attrs": { "color": "red", "size": 42 } }

The row is edited to drop color, and the sink sends:

{ "id": "1", "name": "Shoe", "attrs": { "size": 42 } }

attrs.color is still "red" in the index, and attrs.color:=red still
matches it.

action=upsert replaces the whole document and is what CDC semantics want, but
it's unreachable: params: %{action: "emplace"} is a literal in
lib/sequin/sinks/typesense/client.ex, and Consumers.TypesenseSink has no
action field for sequin.yml to set.

Emitting explicit nulls instead isn't a general fix as it requires knowing
which keys the stored document has, which a transform over a single CDC row
doesn't.

Ask

An optional sink setting, defaulting to emplace so nothing changes for
existing users:

destination:
  type: typesense
  import_action: upsert   # default: emplace

Sequin v0.14.6, unchanged on main. Happy to open a PR if you're open to it.