Boston Mayor Thomas M. Menino on Tuesday will unveil plans for a $5.5 million innovation center in the Seaport District, part of a broader effort to transform the area into a hotbed of entrepreneurship and invention. The facility will provide space for promising companies and executives to meet and exchange ideas, spurring economic renewal in a rapidly growing corner of the city. It will be built by private developers and operated by the Cambridge Innovation Center, an organization that now provides low-cost office space for 450 start-up companies in Kendall Square.