By Gertrude Chavez-Dreyfuss and Elizabeth Howcroft NEW YORK/LONDON (Reuters) – With major cryptocurrency exchange FTX on the brink of collapse, some investors are beginning to question the viability of a sector already bruised by the bitcoin bubble bursting and closures of key market players. Crypto markets have come under intense pressure this year, as rising interest rates prompt investors to ditch risky or speculative assets. The collapse of several crypto lenders, including Celsius and Voyager, major tokens terraUSD and Luna, and hedge fund Three Arrows Capital, had rung alarm bells even b…