Implement the `vFile:fstat` packet

[andrurogerz]

The vFile:fstat packet isn't documented with the other platform packets, but it was added to lldb in 2021.

There is a test for it that currently fails when run against ds2 on Linux or Android: TestGdbRemotePlatformFile.TestGdbRemotePlatformFile.test_platform_file_fstat_llgs