feat(binding/c): add is_exist to operator

[xyjixyjixyji]

• Add is_exist operation to check whether the path exists, which is required in BDD tests.
  • The test for is_exist will be added when we support the Fs backend.

[suyanhanx]

We want binding to be like a native library. We may shouldn't add primitive types like Vec.

[xyjixyjixyji]

We want binding to be like a native library. We may shouldn't add primitive types like Vec.

This only expose a pointer to vector. And all APIs are exposed by us.

If we do not expose types like Vec, and only use data+len way, we are going to face following problems.

• The biggest problem is that the opendal_bytes will be extremely constraint, meaning that the user cannot operate further on this contiguous memory space, the only operation on opendal_bytes will be read and truncate.
• We can not utilize the Rust optimization on Vec.

[xyjixyjixyji]

I will change the naming of is_exist and revert the changing on opendal_byte since it is not needed for us.

[Xuanwo]

I will change the naming of is_exist and revert the changing on opendal_byte since it is not needed for us.

Thank you very much for your effort and patience. I learned a lot about C programming!

[Xuanwo]

Great, thanks a lot!