[common] Add truncation to arbitrary size to Protected Files

[dimakuv]

Description of the changes

Truncation of PFs to arbitrary size is implemented in a slow and trivial way: read the file's contents into a temporary buffer, shrink the file to zero, write back the contents into the file.

Fixes #950.

How to test this PR?

CI. I added tests to libos/test/fs.

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This change is 

[dimakuv]

We hit this again on a ~~MariaDB~~ MongoDB workload -- without this PR it fails with ftruncate: Permission denied on a Protected File. With this PR, it succeeds initialization and continues working.

We should prioritize finishing this PR.