pkg_deb: add installed_size_file attribute

[chouquette]

Lets installed_size be supplied from a file, so callers can compute it themselves.

This PR introduces a new installed_size_file attribute to pkg_deb. It's mutually exclusive with the existing installed_size, and can be used to provide the Installed-Size field from a file generated by a prior action.

This will likely supersede #1079

[chouquette]

From #1079:

I think what I would rather see is the size (and checksums?) being a side effect output the tarball maker.
That eliminates a second pass over the file. Let's prototype that in datadog-agent, and then upstream what works.

DataDog/datadog-agent#57156