sql: allow DEFAULT/ON UPDATE types that can be assignment-cast to column type

[mgartner]

Postgres allows creating a DEFAULT column expression of a type that is different from the column's type, as long as the type of the DEFAULT expression can be assignment-cast to the column type. CRDB does not currently allow this.

Example: FLOAT4 -> INT4 casts are allowed in assignment contexts

In Postgres:

marcus=# CREATE TABLE t (a INT4 DEFAULT 1.0::FLOAT4);
CREATE TABLE

In CRDB:

defaultdb> CREATE TABLE t (a INT4 DEFAULT 1.0::FLOAT4);
ERROR: expected DEFAULT expression to have type int4, but '1.0::FLOAT4' has type float4

We also support ON UPDATE expressions (which Postgres does not) and these should also allowed to be typed as any type that can be assignment-cast to the column's type.

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We must take care to disallow altering a column to a type which cannot be assignment-casted from the types of the DEFAULT and ON UPDATE expressions.

Example: FLOAT4 -> BOOL casts are NOT allowed in assignment contexts

In Postgres:

marcus=# CREATE TABLE t (a INT4 DEFAULT 1.0::FLOAT4);
CREATE TABLE

marcus=# ALTER TABLE t ALTER COLUMN a TYPE BOOL USING a::BOOL;
ERROR:  42804: default for column "a" cannot be cast automatically to type boolean

Jira issue: CRDB-12293

[mgartner]

Related context: #70605 (comment)