Support default value for `any` type

**Describe the problem/challenge you have**

I want set the default value for `any` type but I can't.

Here is the example.

- schema.yaml

```yaml
#@data/values-schema
---
test:
#@schema/type any=True
optional_config:
param1: "a"
param2: "b"
```

- template.yaml

```yaml
#@ load("@ytt:data", "data")

conf: #@ data.values.test.optional_config
```

- value.yaml

```yaml
#@data/values
---
test:
optional_config:
param3: "c"

```

```shell
$ ytt -f value.yaml -f schema.yaml -f template.yaml
ytt: Error: Overlaying data values (in following order: value.yaml):
Document on line value.yaml:2:
Map item (key 'test') on line value.yaml:3:
Map item (key 'optional_config') on line value.yaml:4:
Map item (key 'param3') on line value.yaml:5:
Expected number of matched nodes to be 1, but was 0
```

**Describe the solution you'd like**

In the example, the expected output is:

```yaml
conf:
param1: a
param2: b
param3: c
```

I hope this is supported 🙏

Or I would be glad if you could share the workarounds for this 🙇‍♂️

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