feat(table helpers): allow removing default table helpers

[Cih2001]

This will let users to remove default table helpers. I have something like the following in my neovim config:

vim.cmd([[
let g:db_ui_table_helpers = {
\   'postgresql': {
\     'List': "select * from {schema}.{table} limit 2;",
\     'Columns': "",
\     'Primary Keys': "",
\     'Indexes': "",
\     'References': "",
\     'Foreign Keys': "",
\     'Info': "select format('%s.%s', table_schema, table_name) as tablename,  column_name, column_default, is_nullable, data_type\n
\from information_schema.columns where table_name='{table}' and table_schema='{schema}' order by ordinal_position asc;\n
\select format('%s.%s', schemaname, tablename) as tablename, indexname, indexdef from pg_indexes where tablename='{table}' and schemaname='{schema}';\n
\select conrelid::regclass as tablename, conname, pg_get_constraintdef(oid) from pg_constraint\n
\where conrelid::regclass::text = '{schema}.{table}' order by contype desc;\n
\select conrelid::regclass as dependancies, conname, pg_get_constraintdef(oid) from pg_constraint\n
\where confrelid::regclass::text = '{schema}.{table}'order by contype desc;"
\   }
\ }
]])

Which removes the default helpers I don't use.