Travelers are gaining access to places and experiences across Africa once closed off or just unknown to them. JOHANNESBURG, SOUTH AFRICA — Nearly two years ago, Mengesha Tesfaye was scrambling to make ends meet shining shoes in Lalibela, a picturesque Ethiopian town known for its medieval-era rock-hewn churches. Then, one morning, he struck up a conversation with a French tourist, who had a strange idea for how he could earn a bit of extra cash.…