[vminitd]: api for trim filesystem operations

[saehejkang]

Addition of vminitd API for trim filesystem operations

Closes #1402

Depends on #685

[saehejkang]

Kind of messy, as the work is added to the freeze/thaw filesystem PR, but ready for initial review

[jglogan]

For FITRIM I don't think we want a 1:1 mapping between the ioctl parameters and the vminit API parameters. At least for now, we want to trim the entire volume that path points to:

struct fstrim_range range = {
    .start = 0,
    .len = ULLONG_MAX,
    .minlen = 0,
};

[jglogan]

What we want to build toward for this call is something where we define trim policies. For this PR, schedule could just contain OneShot, and we could add Recurring in a follow-up PR.

Once the one-shot operation is merged, you can implement a container clean command that runs one shot trim on the container root fs path, and the mount paths for each named volume.

Recurring would trim the named path periodically, with a duration of zero clearing a previously established schedule for the path.

Discussion topics once we get to the recurring trim policy include:

• Should the call naively trim path, or should it infer the mount point such that callers don't inadvertently install multiple periodic trims that operate on the same mount?
• Should there be a Params value that clears all previously established trim policies?

import "google/protobuf/duration.proto";

message FiTrimParams {
  oneof schedule {
    OneShot one_shot = 1;
    Recurring recurring = 2;
  }

  message OneShot {}

  message Recurring {
    google.protobuf.Duration interval = 1;
  }
}