[Feature] Add base64gzip function support to Packer template

[pwfee]

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

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[lbajolet-hashicorp]

Hi @pwfee,

Thanks for the PR, this LGTM! I left a small nit on a comment, but that's minor, I'll let you address that, and I'll come back for a final pass of review after.

Pre-approving to not block later.

[lbajolet-hashicorp]

Thanks for the update @pwfee, merging once tests go green

[lbajolet-hashicorp]

Only test failing is the Vercel deployment, which is irrelevant to this PR, so merging now.

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