Introduces a new --prebuilt <source_type> flag. This flag is mutually
exclusive with the existing --tools-file flag.
Added a new directory cmd/prebuiltconfigs/ to store the prebuilt
tools.yaml files (e.g., alloydb.yaml, postgres.yaml, etc.).
These YAML files are embedded into the Go binary using the //go:embed
directive.
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Co-authored-by: Averi Kitsch <akitsch@google.com>
