[seeker] introduce new otSeeker APIs (#12357)

This commit introduces Seeker APIs in OpenThread. An earlier commit
extracted the discovery and candidate selection logic from the `Joiner`
role into a new, standalone `Seeker` module.

The `Seeker` is responsible for performing MLE Discover Scans to find
nearby Joiner Router candidates. It prioritizes these candidates based
on RSSI and steering data (indicating whether the Joiner is preferred)
and manages the list of candidates for connection attempts.

This separation allows the `Seeker` functionality to be utilized
independently of the full `Joiner` role, enabling the development of
custom joining mechanisms over Thread.

A new configuration option `OPENTHREAD_CONFIG_SEEKER_ENABLE` has been
added to control the presence of `otSeeker` APIs.

doc/ot_api_doc.h

@@ -128,6 +128,7 @@
  * @brief Includes functions for the Operational Dataset API.
  * @defgroup api-thread-router           Router/Leader
  * @brief This module includes functions for Thread Routers and Leaders.
+ * @defgroup api-seeker                  Seeker
  * @defgroup api-server                  Server
  * @defgroup api-steering-data           Steering Data
  *