PS-11173: Declarative init.groovy.d boot loading for Terraform-managed masters

[nogueiraanderson]

Feature

• Adds init_groovy_hooks (map of filename to pinned raw URL) to the jenkins-master module; user-data fetches each into init.groovy.d at boot, generalizing the single-file plugin_install_hook. Wires ps3 with its full set (cloud, matrix, durability, hetznerArmHealth, ec2FleetCloud + htz.cloud from the hetzner branch).

Why

• A TF master's init.groovy.d wiring currently lives only on the carried-forward EBS data volume; a fresh-volume rebuild comes up bare (the bootstrap gap only pxc.cd escaped). This makes the wiring declarative-in-TF and self-healing on rebuild.
• ps3 surfaced this concretely: its EC2FleetCloud was in-memory-only and vanished on a JVM restart. On-disk + boot-loaded is the durable fix; jenkins iac deploy stays the no-restart hot-reload path for live edits.
• Scales to the CFN-to-TF migration: each master declares its hooks and the same boot loop self-heals it. A failed fetch warns, never blocks boot.

Tickets

• PS-11173 (master resilience), PS-11179 (the EC2FleetCloud wiring this loads).

[nogueiraanderson]

Validation

• tofu validate: Success (template directives + the map(string) var typecheck clean).
• Scoped plan (-target=module.ps3.aws_launch_template.master, percona-dev-admin): one in-place update to the launch template (user_data to a new version), no instance replacement. ignore_changes = [user_data] was already lifted in PS-11173 Phase 1, so the new wiring rides the next launch / fresh-volume rebuild without churn.
• The on-disk + boot-load path itself was already proven on ps57 this week (the jenkins iac deploy --branch of ec2FleetCloud.groovy registered + survived, end-to-end Graviton allocation validated).

Before merge / apply

• ps3_init_groovy_ref is a master placeholder. Pin it to the merge commit of jenkins-pipelines PR 4126 + PR 4129 before relying on this for DR: until those land, durability.groovy and ec2FleetCloud.groovy are absent on master, so the boot loop warns (without blocking) for those two.
• htz.cloud.groovy is pulled from the hetzner branch (the two-IaC-branch split); pin that to a commit before trusting it for a fresh-volume rebuild.

Scope

• plugin_install_hook is unchanged (back-compat); init_groovy_hooks is additive and {}-default, so every other master is a no-op until it opts in. This is the generalized pxc bootstrap pattern, ready for the CFN-to-TF migration to adopt per master.