add PMG::DKIMSign module

add PMG::DKIMSign module

the module serves 3 purposes:
* it extends Mail::DKIM::Signer:
  * it provides a glue layer between MIME::Entity's output method (which
    expects print and uses \n as line terminator) and Mail::DKIM::Signer's
    PRINT method (which expects \r\n)

* it integrates with PMG's config
  * the domain which should be used for signing is selected based on the
    sender's e-mail address and the DKIM-settings in PMG-configuration
  * it provides a method which takes a MIME::Entity and returns it with
    signature
  * certain headers get oversigned (in order to prevent adding a previously
    non-existing header (e.g. Reply-To) and retaining a valid signature).
    the list of headers which are oversigned is inspired by rspamd's choice [0].
    for rationale see [1,2]

* it provides methods for handling selectors and keys.

[0] https://rspamd.com/doc/modules/dkim_signing.html#sign-headers
[1] https://noxxi.de/research/breaking-dkim-on-purpose-and-by-chance.html
[2] https://github.com/rspamd/rspamd/issues/2136

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>