By Suzanne McGee (Reuters) – Crypto enthusiasts hailed the approval of U.S. bitcoin exchange-traded funds as the birth of a new asset class, but broadening acceptance of the famously turbulent cryptocurrency beyond its true believers could prove challenging. A decade in the making, the ETFs offer investors access to spot bitcoin prices, avoiding the risks associated with holding bitcoin directly in a digital wallet, ranging from hacking to fraudulent behavior by crypto exchanges. Estimates of likely first-year inflows vary widely, from $5 billion to $100 billion. Some market participants have …