This commit moves `encodeUrl` and `registerUrlEncoder` from
ServerHttpResponse to ServerWebExchange.
It also renames `encodeUrl` to `transformUrl` and `registerUrlEncoder`
to `addUrlTransformer` to make it clearer that these methods do not
perform actual URL encodings (i.e. they do not replaceinvalid
characters).
The `add` prefix (instead of `register`) makes it clearer that each
function is added in addition to the previous one.
Issue: SPR-15924
02a2c400c7