feat: allow rock-pull-request to work with monorepo, concierge installed snaps configurable

[shayancanonical]

Issue

We would like to use the rock-pull-request.yaml workflow to test the Temporal rocks.

1. The Temporal rocks repo is a monorepo (with all the Temporal rocks hosted in one repository). The rock-pull-request workflow assumes that the github repo hosts only one rock project.
2. The rock-pull-request installs a fixed set of snaps into the concierge env where the rock tests are executed. This is a limitation, as the Temporal rocks require some additional snaps installed that the justfile targets use.

Solution

1. Adjust the workflows to work with version directories (output of changes job) of the form 1.23.1 as well as temporal-server/1.23.1. This former will ensure that observability rock repo tests continue to work, whereas the latter will ensure that the Temporal rocks monorepo is compatible with this workflow.
2. Add input extra-snaps which can be used to configure the snaps installed into the concierge env. Defaults to rockcraft,just,kubectl

Testing

Test run in the Temporal rocks repo: https://github.com/canonical/temporal-rocks/actions/runs/15983452446/job/45131737223

[sed-i]

The concierge step below is impl detail that the rest of the workflow depends on. We would like to avoid external args breaking it. Options:

• Keep the concierge command as is, and follow up with a "raw" snap install.
• Append the input here to the concierge arg below.

Wdyt?