Add print-schema option allow_tables_in_same_query

[sgoll]

This adds a new config option to print-schema, i.e. allow_tables_in_same_query (in diesel.toml) or --allow-tables-in-same-query (in CLI).

Setting this option to all_tables, or leaving it unset, remains the current behavior of generating a single invocation of the macro allow_tables_to_appear_in_same_query! that lists all tables in the schema.

Setting this option to fk_related_tables changes this behavior: instead of a single invocation, several invocations are issued, one for each group of tables related to each other through foreign keys.

For example, given the following tables with foreign keys:

users (id)
posts (id, user_id -> users(id))
comments (id, post_id -> posts(id))

sessions (id)
transactions (id, session_id -> sessions(id))

cars (id)
bikes (id)

The following macro invocations are generated:

For all_tables (default):

diesel::allow_tables_to_appear_in_same_query!(
    bikes, cars, comments, posts, sessions, transactions, users);

For fk_related_tables:

diesel::allow_tables_to_appear_in_same_query!(comments, posts, users);
diesel::allow_tables_to_appear_in_same_query!(sessions, transactions);

See #4333 for rationale.

[sgoll]

@weiznich When this gets merged, I can also provide a backport for #4490, or for a future release.