Decode blocks into map with custom struct types

[notnoop]

HCLv2 gohcl supports parsing labeled blocks into slices (e.g. []customStruct). This adds support for decoding labeled blocks into map types (e.g. map[string]customStruct), with the additional constraint that labels are unique.

We special case single labeled blocks, as the behavior for multi labeled blocks is not well scoped.

This i s motivated by some map typed fields in Nomad structure (like TaskGroup.Volumes, and ConsulGatewayProxy. EnvoyGatewayBindAddresses. While I suspect this is of wider benefit, I understand if the use case is deemed to narrow to include into the base gohcl library.

Concretely, assuming a type Policies map[string]customStruct 'hcl:"policy,block"', the following hcl declaration

policy "foo" {
  val = "a"
}
policy "bar" {
  val = "b"
}

is decoded as

map[string}*customStruct{
  "foo": &customStruct{Val: "a"},
  "bar": &customStruct{Val: "b"},
}

[jackbackes]

Is there any movement on this PR? I have a need for supporting labeled blocks, and the workaround is... ugly.

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