Add Debian package metadata, vendored deps, and CI configuration

.gitlab-ci.yml

debian/changelog

debian/compat

debian/control

debian/copyright

debian/dovecot-exporter.default

debian/dovecot-exporter.service

debian/rules

debian/source/format

vendor/github.com/alecthomas/template/LICENSE

vendor/github.com/alecthomas/template/README.md

vendor/github.com/alecthomas/template/doc.go

vendor/github.com/alecthomas/template/exec.go

vendor/github.com/alecthomas/template/funcs.go

vendor/github.com/alecthomas/template/helper.go

vendor/github.com/alecthomas/template/parse/lex.go

vendor/github.com/alecthomas/template/parse/node.go

vendor/github.com/alecthomas/template/parse/parse.go

vendor/github.com/alecthomas/template/template.go

vendor/github.com/alecthomas/units/COPYING