chore: add deployment config option to append custom csp directives (#15596)

Allows adding custom static CSP directives to Coder. Niche use case but
makes this easier then creating a reverse proxy that has to replace the
header. We want to preserve our directives, so having an append option
is preferred to a "replace" option via a reverse proxy.


Closes https://github.com/coder/coder/issues/15118

cli/testdata/coder_server_--help.golden

@@ -294,6 +294,13 @@ backed by Tailscale and WireGuard.
           + 1`. Use special value 'disable' to turn off STUN completely.
 
 NETWORKING / HTTP OPTIONS: 
+      --additional-csp-policy string-array, $CODER_ADDITIONAL_CSP_POLICY
+          Coder configures a Content Security Policy (CSP) to protect against
+          XSS attacks. This setting allows you to add additional CSP directives,
+          which can open the attack surface of the deployment. Format matches
+          the CSP directive format, e.g. --additional-csp-policy="script-src
+          https://example.com".
+
       --disable-password-auth bool, $CODER_DISABLE_PASSWORD_AUTH
           Disable password authentication. This is recommended for security
           purposes in production deployments that rely on an identity provider.

cli/testdata/server-config.yaml.golden

@@ -16,6 +16,12 @@ networking:
     # HTTP bind address of the server. Unset to disable the HTTP endpoint.
     # (default: 127.0.0.1:3000, type: string)
     httpAddress: 127.0.0.1:3000
+    # Coder configures a Content Security Policy (CSP) to protect against XSS attacks.
+    # This setting allows you to add additional CSP directives, which can open the
+    # attack surface of the deployment. Format matches the CSP directive format, e.g.
+    # --additional-csp-policy="script-src https://example.com".
+    # (default: <unset>, type: string-array)
+    additionalCSPPolicy: []
     # The maximum lifetime duration users can specify when creating an API token.
     # (default: 876600h0m0s, type: duration)
     maxTokenLifetime: 876600h0m0s