gpg-agent: make shell integrations optional

[crazymanjinn]

Description

In esoteric setups, automatically setting GPG_TTY to current tty is not
desired on every shell startup. This change adds configuration options
to allow user to disable that if desired.

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all.

Appears an unrelated test case is failing even before my change:

❯ nix-shell --pure tests -A run.all
error: attribute 'shellDryRun' missing

       at /home/user/projects/home-manager/modules/programs/bash.nix:12:9:

           11|     checkPhase = ''
           12|       ${pkgs.stdenv.shellDryRun} "$target"
             |         ^
           13|     '';
(use '--show-trace' to show detailed location information)

However, all the gpg-agent related tests do pass:

❯ for t in gpg-agent-default-homedir gpg-agent-override-homedir; do nix-shell --pure tests -A run.$t; done
❯ for t in gpg-agent-default-homedir gpg-agent-override-homedir; do nix-shell --pure tests -A run.$t; done
gpg-agent-default-homedir: OK
gpg-agent-override-homedir: OK

• Test cases updated/added. See example.

Change seems trivial enough it shouldn't need new testcases? Happy to be wrong, so please let me know if you would like me to add some.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

This PR does not.