Correct implementation of `sql-set-product-feature' (Bug#30494).

Correct implementation of `sql-set-product-feature' (Bug#30494).

* lisp.progmodes/sql.el (sql-add-product): Correct argument spec.
(sql-set-product-feature): Handle all cases as intended.
(sql-get-product-feature): Fetch varaiable value by `eval'.
* test/lisp/progmodes/sql-tests.el (sql-test-feature-value-[a-d]):
New test variables.
(sql-test-product-feature-harness): New test macro.
(sql-test-add-product, sql-test-add-existing-product)
(sql-test-set-feature, sql-test-set-indirect-feature)
(sql-test-set-existing-feature)
(sql-test-set-existing-indirect-feature)
(sql-test-set-missing-product, sql-test-get-feature)
(sql-test-get-indirect-feature, sql-test-get-missing-product)
(sql-test-get-missing-feature)
(sql-test-get-missing-indirect-feature): New ERT tests