feat: Prevent lambda-promtail from recompiling on every plan

[mchaffee-anaconda]

What this PR does / why we need it:

Currently, if you deploy lambda-promtail using the provided Terraform code, every time you run terraform plan, the golang binary will be recompiled and you will never have a clean plan. Ideally, the binary should only be recompiled if the code actually changes compared to the last apply.

If the code was just a single file, we could use a hash of the script as the trigger, but we have too many files to make hashing feasible. Terraform won't let you hash every file in a folder natively, you'd have to add a third-party module or write a script yourself.

So instead, I've added a new variable, lambda_code_version, which if set, will be used as the trigger for determining when the code should be recompiled. I've added an example to the README.

Special notes for your reviewer:

n/a

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

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