Description of problem:
Currently it seems to me there's no way to get the package rpm header through the dnf API. To provide feature parity of both yum and dnf, please add the hdr property to the Package class (https://dnf.readthedocs.io/en/latest/api_package.html#dnf.package.Package). In yum, it is available in the RPMInstalledPackage class.

Version-Release number of selected component (if applicable):
dnf-0:4.2.17-6.el8.noarch
rpm-0:4.14.2-37.el8.x86_64

Steps to Reproduce:
RHEL 7.8:
```
import yum
yum_base = yum.YumBase()
hdr = yum_base.rpmdb.returnPackages()[0].hdr
pkg_sig = hdr.sprintf('%|DSAHEADER?{%{DSAHEADER:pgpsig}}:{%|RSAHEADER?{%{RSAHEADER:pgpsig}}:{(none)}|}|')
print(pkg_sig)
```
Output: "RSA/SHA256, Wed Feb 27 12:12:40 2019, Key ID 199e2f91fd431d51"

RHEL 8.2
```
import dnf
dnf_base = dnf.Base()
dnf_base.fill_sack(load_system_repo=True, load_available_repos=False)
dnf_base.sack.query().installed()[0].hdr
```
Output: a traceback "AttributeError: 'Package' object has no attribute 'hdr'"

Actual results:
I'm unable to get the rpm header of a package through the dnf python module.

Expected results:
I have a way to get the rpm header of a package through the dnf python API.

Additional notes:
The Convert2RHEL tool needs to be able to read the rpm header to get the signature of an installed package on CentOS 8.