This commit adds assertions to MockMvc's CookieresultMatchers:
- `attribute` for arbitrary attributes
- `sameSite` for the SameSite well-known attribute
Note that the `sameSite` methods delegate to their `attribute`
counterparts. Note also that Jakarta's `Cookie#getAttribute` method is
case-insensitive, which is reflected in the documentation of the
`attribute` assertion method and the tests.
Closes gh-30285
d6460e0d57