[Validator] Support "maxSize" given in KiB

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.6
| Fixed tickets | symfony/symfony#10962


To display the constraint validation message with an expected suffix, this PR add a new option in  File constraint.
Are the `kibibytes` and `mebibytes` support in `maxSize` also new in 2.6? If so, we need a `versionadded:: 2.6` for them as well.

What do you all think about replacing the bullet points with a table of examples? Something like:

| maxSize  | Notes | Unit Name |
| ------------- | -------- | --------- |
| 4096  | 4096 bytes | bytes |
| 200k  | 1k = 1,000 bytes | kilobye |
| 4M  | 1M = 1,000,000 bytes | megabyte |

etc