Retrieve entire map of secrets with vault function

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#### Description

Currently, [`vault` function](https://www.packer.io/docs/templates/hcl_templates/functions/contextual/vault) allows to retrieve a single key from a key-value map at a given secret path. It would be great if it would allow to retrieve the entire map of key/value pairs (e.g. simply by omitting the second argument).

#### Potential configuration

Instead of pulling out a single value with
```
locals {
foo = vault("/secret/data/hello", "foo")
}
```
we could pull out the entire map with e.g.
```
locals {
hello_vars = vault("/secret/data/hello")
}
```
so `hello_vars` would be a map with all the key-value pairs.