feat(core): change Result to default error

[George-Miao]

This PR changes Result<T> = std::result::Result<T, Error> to Result<T, E = Error> = std::result::Result<T, E>, makes it easier for user to use even when they have non-opendal errors.

Example

use opendal::Result;

// This still works
let a: Result<T>;

// And this works too
let b: Result<T, MyError>;

[Xuanwo]

Thanks! I believe we can replace all current std::result::Result changes back to opendal::Result now. Would you like to create an issue for this?

[George-Miao]

Thanks! I believe we can replace all current std::result::Result changes back to opendal::Result now. Would you like to create an issue for this?

Sure