feat: custom flag parser for Platform args

[jedevc]

This is tricky, since the scalar aliases were getting lost in our TypeDef code. We can preserve these, and then provide a custom parser.

With this, we can add validation that the target platform is valid, as well as allow a special current platform, which allows this use case:

package main

import (
	"fmt"
)

type Playground struct{}

func (m *Playground) Hello(platform Platform) string {
	return fmt.Sprintf("hello i am platform %q", platform)
}

❯ dagger call hello --platform=current
hello i am platform "linux/amd64"

[helderco]

I'd rather have a proper ScalarKind (and EnumKind) for core translation. See DEV-3403.

[jedevc]

@helderco 100% agree, this was just a WIP to see if the idea worked 🎉

Next step is a proper ScalarKind!

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