Avoid unnecessary escaping in 'url-build-query-string'

Avoid unnecessary escaping in 'url-build-query-string'

* lisp/url/url-util.el (url-query-key-value-allowed-chars):
New defconst.
(url-build-query-string): Use it to escape only those
characters that need it in keys and values.
* test/lisp/url/url-util-tests.el (url-util-tests): Add
new test cases.  (Bug#70312)

(cherry picked from commit 40629706b1ff1a2a596420dd2ac388b7e5fc656c)