fatfs: fstat - enable setting a custom preferred block size

Enables setting a custom st_blksize in fatfs vfs fstat function.
Directly affects file buffer size for fatfs.
Increasing the value helps with fread and fgets speeds, however increases heap usage.
Added info to docs about improving I/O performance.

components/fatfs/vfs/vfs_fat.c

@@ -600,6 +600,7 @@ static int vfs_fat_fstat(void* ctx, int fd, struct stat * st)
     st->st_mtime = 0;
     st->st_atime = 0;
     st->st_ctime = 0;
+    st->st_blksize = CONFIG_FATFS_VFS_FSTAT_BLKSIZE;
     return 0;
 }