sql/schemachanger: support placeholder arguments in declarative schema changer

**Describe the problem**

The declarative schema changer does not have good support for placeholders in SQL statements.

**To Reproduce**

Running on 8c5af013aa7

```
root@localhost:26257/defaultdb> create table foo (a int);
CREATE TABLE

root@localhost:26257/defaultdb> set use_declarative_schema_changer = unsafe_always;
SET

root@localhost:26257/defaultdb> prepare q as ALTER TABLE foo CONFIGURE ZONE USING gc.ttlseconds = 1, range_max_bytes = $1, range_min_bytes = 1<<10;
ERROR: internal error: building declarative schema change targets for CONFIGURE ZONE: expected *DInt, found *tree.Placeholder
SQLSTATE: XX000
DETAIL: stack trace:
pkg/sql/sem/tree/datum.go:691: MustBeDInt()
pkg/sql/catalog/zone/zones.go:65: func2()
pkg/sql/schemachanger/scbuild/internal/scbuildstmt/zone_config_helpers.go:388: func1()
pkg/sql/schemachanger/scbuild/internal/scbuildstmt/zone_config_helpers.go:462: func1()
pkg/sql/schemachanger/scbuild/internal/scbuildstmt/zone_config_helpers.go:465: loadSettingsToZoneConfigs()
pkg/sql/schemachanger/scbuild/internal/scbuildstmt/zone_config_helpers.go:1183: prepareZoneConfig()
pkg/sql/schemachanger/scbuild/internal/scbuildstmt/table_zone_config.go:201: applyZoneConfig()
pkg/sql/schemachanger/scbuild/internal/scbuildstmt/configure_zone.go:70: SetZoneConfig()
GOROOT/src/reflect/value.go:581: call()
GOROOT/src/reflect/value.go:365: Call()
pkg/sql/schemachanger/scbuild/internal/scbuildstmt/process.go:192: Process()
pkg/sql/schemachanger/scbuild/build.go:102: Build()
pkg/sql/schema_change_plan_node.go:70: SchemaChange()
pkg/sql/opaque.go:52: buildOpaque()
pkg/sql/opt/optbuilder/opaque.go:65: tryBuildOpaque()
pkg/sql/opt/optbuilder/builder.go:506: buildStmt()
pkg/sql/opt/optbuilder/builder.go:323: buildStmtAtRootWithScope()
pkg/sql/opt/optbuilder/builder.go:304: buildStmtAtRoot()
pkg/sql/opt/optbuilder/builder.go:283: Build()
pkg/sql/plan_opt.go:494: buildReusableMemo()
pkg/sql/plan_opt.go:173: prepareUsingOptimizer()
pkg/sql/conn_executor_prepare.go:339: populatePrepared()
pkg/sql/conn_executor_prepare.go:279: func2()
pkg/sql/conn_executor_prepare.go:284: prepare()
pkg/sql/conn_executor_prepare.go:117: addPreparedStmt()
pkg/sql/conn_executor_exec.go:895: execStmtInOpenState()
pkg/sql/conn_executor_exec.go:165: func2()
pkg/sql/conn_executor_exec.go:4428: execWithProfiling()
pkg/sql/conn_executor_exec.go:164: execStmt()
pkg/sql/conn_executor.go:2424: func1()
pkg/sql/conn_executor.go:2429: execCmd()
pkg/sql/conn_executor.go:2346: run()
```

Here is a different example of a statement that can be prepared, but then fails at execution time.
```
root@localhost:26257/defaultdb> create table foo (a int);
CREATE TABLE

root@localhost:26257/defaultdb> set use_declarative_schema_changer = unsafe_always;
SET

root@localhost:26257/defaultdb> prepare q as alter table foo add column b int default $1:::int;
PREPARE

Time: 1ms total (execution 0ms / network 0ms)

root@localhost:26257/defaultdb> execute q(3::int);
ERROR: internal error: no type for placeholder $1
SQLSTATE: XX000
DETAIL: stack trace:
pkg/sql/execute.go:38: fillInPlaceholders()
pkg/sql/conn_executor_exec.go:523: execStmtInOpenState()
pkg/sql/conn_executor_exec.go:165: func2()
pkg/sql/conn_executor_exec.go:4428: execWithProfiling()
pkg/sql/conn_executor_exec.go:164: execStmt()
pkg/sql/conn_executor.go:2424: func1()
pkg/sql/conn_executor.go:2429: execCmd()
pkg/sql/conn_executor.go:2346: run()
pkg/sql/conn_executor.go:1017: ServeConn()
pkg/sql/pgwire/conn.go:252: processCommands()
pkg/sql/pgwire/server.go:1197: func4()
src/runtime/asm_arm64.s:1223: goexit()
```

If `use_declarative_schema_changer` were set to its default value, then the above statements would instead fallback to the legacy schema changer.

**Additional context:**

This was previously reported in https://github.com/cockroachdb/cockroach/issues/126288. That issue was closed but the fix for it did not fully work.

Jira issue: CRDB-48241

Epic CRDB-60938

Reproduce the two prepared-statement examples with the declarative schema changer enabled. Start with pkg/sql/schemachanger/scbuild/internal/scbuildstmt/zone_config_helpers.go and the related stack-trace entry points, then trace placeholder handling through pkg/sql/execute.go and pkg/sql/conn_executor_exec.go. Done means the shown CONFIGURE ZONE and ADD COLUMN statements support placeholder arguments without internal errors.

Written by the indexing model from the issue text.