feat(cik8s) add a new 3x node pool dedicated to the bom builds

[dduportal]

Related to jenkins-infra/helpdesk#3521

This PR adds a new node pool on the EKS cluster cik8s.

The goal is, folllowing experiementations in jenkinsci/bom#1969, to reduce the initial scope and deliver the "resource split" between bom and plugins.

Since increasing the size of VM (to execute more pods at the same time) led to contentions not diasgnosed yet, the choice is to keep the same sizing as we have today.

This node pool is reserved for the bom builds on ci.jenkins.io through the use of taints.
The size of the nodes in this new pool is the same as the current node pool used for both plugins and bom builds.

[boyscout improvements]

• The node label "ci.jenkins.io/agents-density" was missing on the current node pool: it's added (will allow automation of the pod/agents quotas)
• Update naming convention of the current "big" node pool which is unused so can be updated
• Updating the .shared-tools dependency

[dduportal]

Terraform plan looks good to me:

Plan: 12 to add, 1 to change, 6 to destroy.

=> A given node pool is made of 6 terraform resources (identity, policies, etc.).
=> We change 1 node pool (eg. 6 destroy + 6 add) and add 1 node pool (6 add) + the cluster itself to modify