sql: support foreign key references to/from virtual columns

[RaduBerinde]

Jira issue: CRDB-3260

[RaduBerinde]

Fixing #46672 is a prerequisite (or they should be fixed together).

[github-actions]

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[spilchen]

Removing the blocking code is straight forward. However, validating the constraint has issues. This fails when the virtual column is in parent or in the child. Here are two repro scenarios (both of these assume the blocking code is removed):

1. Virtual column in the parent

create table t1 (
  c1 int not null primary key, 
  c21 int not null, 
  c22 int not null, 
  c28 int not null as (c21 + c22) virtual, 
  unique (c28)
);
create table t2 (
  c28 int not null, 
  unique (c28), 
  foreign key (c28) references t1(c28)
);
insert into t1(c1,c21,c22) values (0,0,0);
insert into t2 values (0);
# ERROR: internal error: scanned virtual column depends on non-scanned column. 

2. Virtual column in the child

create table parent (
  a int8 not null, 
  b int8 null primary key
);
create table vchild (
  a int8 not null primary key, 
  computed_col int8 as (a) virtual, 
  foreign key(computed_col) references parent(b)
);
insert into parent values (0,0);
insert into vchild values (0);
update parent set b=10 where b=0;
# ERROR: internal error: scanned virtual column depends on non-scanned column.