feat: custom packager configuration for rpm pkg

[crazy-max]

Atm rpm Packager uses the maintainer value which is sometimes not true when another entity wants to redistribute a package:

#
# Example spec file for cdplayer app...
#
Summary: A CD player app that rocks!
Name: cdplayer
Version: 1.0
Release: 1
License: GPL
Group: Applications/Sound
Source: ftp://ftp.gnomovision.com/pub/cdplayer/cdplayer-1.0.tgz
URL: http://www.gnomovision.com/cdplayer/cdplayer.html
Distribution: WSS Linux
Vendor: White Socks Software, Inc.
Packager: Santa Claus <sclaus@northpole.com>

%description
It slices!  It dices!  It's a CD player app that
can't be beat.  By using the resonant frequency
of the CD itself, it is able to simulate 20X
oversampling.  This leads to sound quality that
cannot be equaled with more mundane software...

The vendor line identifies the organization that distributes the software. Maintaining our fictional motif, we've invented fictional company, White Socks Software, to add to our spec file. Individuals will probably omit this as well.

The packager line is used to identify the organization that actually packaged the software, as opposed to the author of the software. In our example, we've chosen the greatest packager of them all, Santa Claus, to work at White Socks Software. Note that we've included contact information, in the form of an e-mail address.

This PR allows to customize this field and also to expand env var for it.

[codecov]

Codecov Report

Merging #409 (6395411) into main (1de9194) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #409      +/-   ##
==========================================
+ Coverage   66.18%   66.19%   +0.01%     
==========================================
  Files          15       15              
  Lines        1854     1855       +1     
==========================================
+ Hits         1227     1228       +1     
  Misses        488      488              
  Partials      139      139              

Impacted Files	Coverage Δ
nfpm.go	86.29% <100.00%> (+0.06%)	⬆️
rpm/rpm.go	69.05% <100.00%> (ø)

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[crazy-max]

Looks like there is an issue on the Windows runner: https://github.com/goreleaser/nfpm/runs/4370360116?check_suite_focus=true#step:7:8

task: [test] go test  -failfast -race -coverpkg=./... -covermode=atomic -coverprofile=coverage.txt ./... -run . -timeout=5m
# runtime/cgo
cgo: C compiler "gcc" not found: exec: "gcc": executable file not found in %PATH%
FAIL	github.com/goreleaser/nfpm/v2 [build failed]
FAIL	github.com/goreleaser/nfpm/v2/apk [build failed]
FAIL	github.com/goreleaser/nfpm/v2/deb [build failed]
FAIL	github.com/goreleaser/nfpm/v2/deprecation [build failed]
FAIL	github.com/goreleaser/nfpm/v2/files [build failed]
FAIL	github.com/goreleaser/nfpm/v2/internal/glob [build failed]
FAIL	github.com/goreleaser/nfpm/v2/internal/sign [build failed]
FAIL	github.com/goreleaser/nfpm/v2/rpm [build failed]
task: Failed to run task "test": exit status 2
Error: Process completed with exit code 1.

[djgilcrease]

LGTM