Consider the design on top of GNUPGHOME

[wdhongtw]

According to the document from GPG tools, setting GNUPGHOME
can create a separated environment, e.g. socket file path between gpg and gpg-agent.

Currently, this extension communicated with the gpg-agent,
perform a dummy signing operation with pinentry-loopback to force gpg-agent to cache the passphrase.

With GNUPGHOME, it seems possible to do some MITM operation,
so we can prompt user to provide passphrase when it's actually asked by gpg-agent.

vscode -> git -> gpg -redirect by GNUPGHOME-> agent by our extension -> actual agent
                                                       v
                                           ask passphrase from user

Compare to current design, this mechanism need more initial setup and runtime process management.
But it would be a big improve if everything are done properly