sql: support IF NOT EXISTS for CREATE CLUSTER and CREATE CLUSTER REPLICA

[aljoscha]

CREATE CLUSTER and CREATE CLUSTER REPLICA now accept an IF NOT EXISTS
clause, matching the 13 other CREATE statements that already support it.
With it, creating a cluster or replica whose name is already taken succeeds and
emits a cluster "c" already exists, skipping notice (SQLSTATE 42710) instead
of failing, so idempotent provisioning scripts no longer need a pre-flight
existence check.

Skipping leaves the existing object completely untouched. The configuration in
the statement is not applied, so this is not an upsert.

Previously these two statements produced a misleading parse error, because IF
was consumed as the object name and the failure surfaced one token later:

CREATE CLUSTER IF NOT EXISTS c (SIZE '25cc')
ERROR: Expected one of AUTO or AVAILABILITY or DISK or ..., found NOT

Implementation

The machinery already exists end to end, so this is mostly threading a flag:
if_not_exists on the two AST nodes and the two plans, a parse_if_not_exists()
call in each parse function, and the established "turn the catalog's
already-exists error into a notice" arm in the sequencer.

Two things worth calling out:

• The skip happens at the catalog transaction, where every sibling path puts
  it, rather than short-circuiting early on the in-memory catalog. That means
  the statement is validated in full first, so CREATE CLUSTER IF NOT EXISTS c (SIZE 'typo') still reports the bad size rather than silently succeeding
  because c happens to exist. The cost is that a skipped statement still
  allocates (and discards) a cluster id, which is a durable catalog write. That
  is the same cost every other CREATE ... IF NOT EXISTS already pays, and
  silently accepting a typo'd config in a provisioning script is the worse
  failure mode.

• A bare if identifier now renders quoted. The IF [NOT] EXISTS clause sits
  exactly where the object name goes, so a rendered CREATE CLUSTER if (SIZE …)
  no longer reparses, which breaks both the plan/unplan roundtrip assertion in
  plan_create_cluster and SHOW CREATE CLUSTER for a cluster named if.
  This joins the existing list of keywords quoted for exactly this reason
  (PREPARE, INTO, WHEN, …), and it covers the same hazard for every other
  statement whose IF EXISTS clause sits where the name goes. Note the flip
  side: CREATE CLUSTER if … and CREATE CLUSTER REPLICA if.r … used to parse
  (the parser accepts keywords as identifiers) and now do not. Quoting still
  works, and DROP CLUSTER if was already broken the same way.

SHOW CREATE CLUSTER does not render IF NOT EXISTS. It is a property of the
statement, not of the cluster.

Tests

• src/sql-parser/tests/testdata/ddl: syntax for both statements, the
  IF EXISTS typo, and a cluster literally named if.
• test/sqllogictest/cluster.slt: idempotence, that the existing object is left
  untouched, that a bad SIZE is still reported, the managed and unmanaged
  cluster variants, FEATURES, reserved names, case sensitivity, full
  SHOW CREATE CLUSTER output, that the replica form still requires the cluster
  to exist, and that it does not soften the managed-cluster rejection.
• test/pgtest-mz/notice.pt: the wire-level notice for both statements.

Fixes SQL-601

[mgree]

LGTM. It's worth confirming that nobody has an existing cluster named if, which will get borked by this change.

[mgree]

Reading this, I don't have the context to know what a "reconstructed plan" is. Someone who knows this file might, though!

[mgree]

Could this be breaking for existing customers' queries? (Feels crazy to me to use if as a name for something, but that's programmer bias I'm sure.)