Backport of [Feature] Add base64gzip function support to Packer template into release/1.11.x

[hc-github-team-packer]

Backport

This PR is auto-generated from #13142 to be assessed for backporting due to the inclusion of the label backport/1.11.x.

The below text is copied from the body of the original PR.

---

Why

Packer doesn't support base64zip() which is a useful function to compress user_data or any string in Packer. However terraform indeed supports this function.

As one of the many usecases, AWS has limit the EC2 user_data size on 16KB. It's easier to over this size and we are willing to have this compress function.

Changes

• Add base64gzip function support in Packer template
• Add unittest for base64gzip function
• Add mdx page for base64gzip function

Tests

1. Build test: Confirmed we can decode base64gzip from the temporary instance user_data

$ aws ec2 describe-instance-attribute --instance-id i-031779e5c7811745e --attribute userData --output text --query "UserData.Value"  --region us-west-2 | base64 -d | gunzip | head
#cloud-config

# see /etc/cloud/cloud.cfg for order of operations for cloud-init

# This cloud_init will run during the image baking process
# Purpose: Install needed software, pull docker images and provision configs

write_files:
- path: /etc/ssh/sshd_config
  owner: root:root

2. Test base64gzip() in Packer console, confirmed content in base64gzip

> local.user_data
> I2Nsb3VkL....

3. Unittest

---

Overview of commits

• 3198acd - 77e35b9

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.