Add keychain plugin (#7385)

This plugin integrates the Keychain tool[1] in to the project.

[1] [keychain](https://www.funtoo.org/Keychain)

plugins/keychain/README.md

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+# keychain plugin
+
+This plugin starts automatically [`keychain`](https://www.funtoo.org/Keychain)
+to set up and load whichever credentials you want for both gpg and ssh
+connections.
+
+To enable it, add `keychain` to your plugins:
+
+```zsh
+plugins=(... keychain)
+```
+
+**NOTE**: It is HIGHLY recommended to also enable the `gpg-agent` plugin.
+
+## Instructions
+
+**IMPORTANT: put these settings _before_ the line that sources oh-my-zsh**
+
+**To adjust the agents** that keychain manages, use the `agents` style as
+shown below. By default, only the `gpg` agent is managed.
+
+```zsh
+zstyle :omz:plugins:keychain agents gpg,ssh
+```
+
+To **load multiple identities** use the `identities` style, For example:
+
+```zsh
+zstyle :omz:plugins:keychain identities id_ed25519 id_github 2C5879C2
+```
+
+**To pass additional options** to the `keychain` program, use the
+`options` style; for example:
+
+```zsh
+zstyle :omz:plugins:keychain options --quiet
+```
+
+## Credits
+
+Based on code from the `ssh-agent` plugin.
+
+## References
+
+- [Keychain](https://www.funtoo.org/Keychain)

plugins/keychain/keychain.plugin.zsh

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+function _start_agent() {
+	local agents
+	local -a identities
+	local -a options
+	local _keychain_env_sh
+	local _keychain_env_sh_gpg
+
+	# load agents to start.
+	zstyle -s :omz:plugins:keychain agents agents
+
+	# load identities to manage.
+	zstyle -a :omz:plugins:keychain identities identities
+
+	# load additional options
+	zstyle -a :omz:plugins:keychain options options
+
+	# start keychain...
+	keychain ${^options:-} --agents ${agents:-gpg} ${^identities}
+
+	# Get the filenames to store/lookup the environment from
+	_keychain_env_sh="$HOME/.keychain/$SHORT_HOST-sh"
+	_keychain_env_sh_gpg="$HOME/.keychain/$SHORT_HOST-sh-gpg"
+
+	# Source environment settings.
+	[ -f "$_keychain_env_sh" ]     && . "$_keychain_env_sh"
+	[ -f "$_keychain_env_sh_gpg" ] && . "$_keychain_env_sh_gpg"
+}
+
+_start_agent
+
+# tidy up after ourselves
+unfunction _start_agent