Vector: Implement find, find_if, find_first_matching in terms of AK::find*

Problem:
- The implementation of `find` is coupled to the implementation of `Vector`.
- `Vector::find` takes the predicate by value which might be expensive.

Solution:
- Decouple the implementation of `find` from `Vector` by using a
  generic `find` algorithm.
- Change the name of `find` with a predicate to `find_if` so that a
  binding reference can be used and the predicate can be forwarded to
  avoid copies.
- Change all the `find(pred)` call sites to use `find_if`.

Libraries/LibTLS/ClientHandshake.cpp

@@ -398,7 +398,7 @@ ssize_t TLSv12::handle_payload(ReadonlyBytes vbuffer)
                 }
                 payload_res = handle_certificate(buffer.slice(1, payload_size));
                 if (m_context.certificates.size()) {
-                    auto it = m_context.certificates.find([&](auto& cert) { return cert.is_valid(); });
+                    auto it = m_context.certificates.find_if([](const auto& cert) { return cert.is_valid(); });
 
                     if (it.is_end()) {
                         // no valid certificates