secure_storage: use hybrid key store in samples/tests

Use dynamic allocation for key material for
better compatibility as a fully static key store is a new
feature that not all PSA Crypto implementations support.

Explicitly enable CONFIG_MBEDTLS_ENABLE_HEAP to ensure that Mbed TLS uses
heap for the PSA keys' data (instead of failing at runtime).
This will turn off CONFIG_MBEDTLS_PSA_STATIC_KEY_SLOTS, making the
implementation default to dynamic key slots.

Signed-off-by: Tomi Fontanilles <tomi.fontanilles@nordicsemi.no>