xactfetch: Configure secretsocket

The `xactfetch` script now uses a helper tool, `secretsocket` to
handle looking up secrets.  This tool supports various secret source
types, including files, environment variables, and external commands.
Separating this functionality out of the main script makes it a lot
more flexible and pluggable.  It's main purpose, though, was actually
to allow `xactfetch` to run in a container while communicating with
`rbw` outside that container, specifically for development puposes.

The `secretsocket` tool reads its configuration from a TOML document.
This document defines the secrets the tool handles, and how to look
them up.

Note that the `xactfetch` container image no longer defines the
`XDG_CONFIG_HOME` environment variable, as it uses Chromium instead of
Firefox now, and the former does not work with a read-only config
directory.  As such, we have to mount the `rbw` configuration in the
default location.

xactfetch/kustomization.yaml

@@ -17,6 +17,9 @@ configMapGenerator:
 - name: xactfetch
   envs:
   - xactfetch.env
+- name: xactfetch-secrets
+  files:
+  - secrets.toml
 - name: xactfetch-rbw
   files:
   - config.json=rbw-config.json