[feature:gsoc26] Expose certificate data as template variables

[stktyagi]

Is your feature request related to a problem? Please describe.
Once a standalone certificate is generated, its payload, PEM and Private Key, is stored securely in the database. However, this payload is currently inaccessible to the OpenWISP configuration engine, meaning administrators cannot actually deploy the certificates into the device's configuration files.

Describe the solution you'd like
I will extend the configuration variable context to inject certificate payloads directly into the templating engine:

1. Context Generation: implement a get_cert_context() method extending Config.get_context(). This will iterate over the devicecertificate_set and build an Ordered Dictionary of the active certificates.

2. UUID Namespacing: to prevent variable collisions when multiple cert templates are assigned to a single device, generate predictable, UUID-namespaced variables. Specifically:

   • cert_{template_pk_hex}_pem: the generated certificate.
   • cert_{template_pk_hex}_key: the private key.
   • cert_{template_pk_hex}_uuid: the object UUID.

3. Template Usage: administrators will then be able to reference these variables dynamically inside standard OpenWISP templates using the {{ variable_name }} syntax.

Requirement from the GSoC proposal

The proposal requires exposing the certificate PEM, private key PEM, and certificate UUID in Config.get_context() using a UUID-based namespace to avoid conflicts.

Decision to keep open

The proposal example uses names like cert_<template_uuid>_pem, while this issue currently uses cert_{template_pk_hex}_pem. The exact UUID-based namespace should be confirmed during implementation and documented before merging.

Measurable outcomes: 5
Implementation points: 4, 5

[nemesifier]

Updated the issue body to make the GSoC proposal coverage explicit. I added the proposal requirement for certificate PEM, private key PEM, and certificate UUID in Config.get_context(), and left open the namespace mismatch between cert_<template_uuid>_pem and cert_{template_pk_hex}_pem. No design decision was made here.

To review the exact changes, open this issue in the browser, navigate to the top bar of the issue description, click "last edited by nemesifier", then click "most recent". GitHub does not provide a quick URL for that view.