Add support for reading YAML data from remote URLs

[a2ush]

Issue #, if available:

Description of changes:

$ go run cmd/awsdac/main.go https://raw.githubusercontent.com/awslabs/diagram-as-code/main/examples/alb-ec2.yaml -o http-input.png
[Completed] AWS infrastructure diagram generated: http-input.png

$ go run cmd/awsdac/main.go https://github.com/awslabs/diagram-as-code/blob/main/examples/alb-ec2.yaml -o http-input-err.png
FATA[0000] yaml: line 211: mapping values are not allowed in this context 
exit status 1

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