refactor: make ErrorObject::borrowed accept &str

[aj3n]

If you pass DST reference type as message to ErrorObject::borrowed, you'll get error like this:

error[E0277]: the size for values of type `str` cannot be known at compilation time
  --> src/main.rs:2:50
   |
2  |     jsonrpsee::types::ErrorObjectOwned::borrowed(0, "ok", None);
   |     --------------------------------------------    ^^^^ doesn't have a size known at compile-time
   |     |
   |     required by a bound introduced by this call
   |
   = help: the trait `Sized` is not implemented for `str`

It could be more intuitive to accept such usage.

[niklasad1]

@aj3n are you working on making the change as suggested in the code review?

I guess you may need to change a bunch of test code but would be good to merge this.