crosscluster/logical: handle PIT LDR cursor before schema changes

The following is not working as intended.

1. Set `ts = s.Clock().Now()`
2. Add column to table `t`
3. Insert row into `t`
4. Start LDR stream with `t` as target, cursor = `ts`
5. The row added in step 2 is never replicated, and will see something like
```
I260224 18:04:14.748518 6879 crosscluster/logical/logical_replication_job.go:944 [T10,Vtest-tenant,n1,job=LOGICAL REPLICATION id=1153059568756228097] 513 hit retryable error cannot create logical replication stream: destination table tab has 3 columns, but the source table tab has 2 columns
```

We need to handle the case where the descriptor is older than any schema changes so that it can be replicated correctly. If the dest schema has extra columns we insert the default value, if they have less columns we remove them.

Jira issue: CRDB-60736