[meshcop] introduce new public APIs for SteeringData (#12148)

This commit introduces a set of public APIs to allow manipulation
of `otSteeringData`. The new APIs are provided when the configuration
`OPENTHREAD_CONFIG_MESHCOP_STEERING_DATA_API_ENABLE` is enabled.

The internal `SteeringData` is also improved to enhance robustness.
Methods such as `Init()`, `UpdateBloomFilter()` now return an `Error`
to signal failures on invalid arguments (e.g., invalid length)
instead of asserting.

doc/ot_api_doc.h

@@ -129,6 +129,7 @@
  * @defgroup api-thread-router           Router/Leader
  * @brief This module includes functions for Thread Routers and Leaders.
  * @defgroup api-server                  Server
+ * @defgroup api-steering-data           Steering Data
  *
  * @}
  *