feat: add cloudwatch_logs_log_group_name variable for log group name override

[supersidor]

NOTE:
I am working in Life360, cant create PR using my Github enterprise user

Adds a new cloudwatch_logs_log_group_name variable that allows overriding the default CloudWatch log group name (/aws/lambda/{function_name}).

This is needed when migrating legacy Terraform configurations to Atmos, where existing Lambda functions may already have CloudWatch log groups with non-standard names. Without this override, the module would
try to recreate the log group with the default naming convention, causing resource conflicts or unnecessary destroy/create cycles during migration.

Changes

• Added cloudwatch_logs_log_group_name variable (string, default null) to variables.tf
• Updated main.tf to use coalesce() to prefer the explicit name when provided, falling back to the default /aws/lambda/${var.function_name}
• Set label_value_case = "none" when a custom name is provided to prevent the cloudposse/cloudwatch-logs/aws module from transforming the log group name with label casing

Usage

module "lambda" {
source = "cloudposse/lambda-function/aws"

# Preserve existing log group name during Atmos migration
cloudwatch_logs_log_group_name = "/aws/lambda/my-legacy-function"

# ... other variables

}

When cloudwatch_logs_log_group_name is not set (default null), behavior is unchanged — the log group defaults to /aws/lambda/{function_name}.