Add truncate operation

- [x] S3
- [x] WebDAV
I would like to change the test case as:
1. read the file
2. truncate file
3. read again
4. check if the (3) result is in the truncated length, and prefix matching to the (1) result.

Then it doesn't rely on the content of big.txt file.
And for the test sanity, I really like to test with a file of the changing content... '1111111111' doesn't explain it comes from the first items, it could be the last 10 bytes.