(feat) Request for checksum_expr field in http backend

[Attempt3035]

Having a checksum_expr field in the http backend the same as version_expr would be fantastic as that would allow dynamic grabbing of the checksum from the same file as the versions are retrieved from. This would for example allow enabling checksum verification for the flutter http installation, using this same file as we get the versions from.

IMO this field should use the same expr-lang semantics as version_expr does, and we should consider additionally including a checksum_json_path and checksum_regex to ensure it's feature complete across both elements.