Add ability to pass headers to the ShapeStream client

[ericallam]

We're going to be proxying through our backend to ElectricSQL and would like to be able to add an authorization header on shape requests without having to pass in a custom fetch implementation that injects headers.

[KyleAMathews]

Nice idea — this is far and away the most common reason for needing a custom fetch implementation so this would be good API sugar.