Support for other PGP implementations and remote keys

It is nice that git-crypt reads the gpg executable from the git config `gpg.program` configuration.
This allows one to interface with other PGP implementations, as soon as the desired commands can be rewritten in terms of `gpg` CLI. For instance, we achieved this with a [fork](https://github.com/fortanix/sq-dsm) of [Sequoia-PGP](https://sequoia-pgp.org/) that allows to keep keys inside an HSM, instead of local keys.

It would be even nicer to either
- be able to interface directly with `sq`
- interface with any implementation of [SOP](https://tools.ietf.org/id/draft-dkg-openpgp-stateless-cli-00.html) (which includes at least 8 PGP implementations, including gpg - see [this](https://tests.sequoia-pgp.org/))

Apologies if this has been already discussed/considered.

Start by reviewing git-crypt's existing handling of the git config gpg.program setting, then compare the command requirements of sq and SOP described in the issue. Done means the project has a decided and tested approach for interfacing with alternative PGP implementations and remote or HSM-held keys.

索引モデルが issue の本文から書いたものです。