`max_path_length` value of 0 in `PkiIssuerSignIntermediateRequest` struct should not be omitted

# Expected Behavior

When signing intermediate certificate with [`client.Secrets.PkiIssuerSignIntermediate`](https://github.com/hashicorp/vault-client-go/blob/main/api_secrets.go#L5017) function with request parameter as:
```go
request := schema.PkiIssuerSignIntermediateRequest{
...
MaxPathLength: 0,
...
}
```
`max_path_length` property is omitted from the request body to the vault API, leading to generated certificate without `pathlength` property:
```
X509v3 Basic Constraints: critical
CA:TRUE, pathlen:0 -> missing part
```

This might be due to `omitempty` setting on `MaxPathLength int32 json:"max_path_length,omitempty"` property in [PkiIssuerSignIntermediateRequest](https://github.com/hashicorp/vault-client-go/blob/main/schema/model_pki_issuer_sign_intermediate_request.go#L38) struct.

# Current Behavior

I think `MaxPathLength: 0` should not be omitted and certificate with `pathlen:0` should be generated.

# Failure Information

v0.4.3

## Steps to Reproduce

Simply try to sign intermediate certificate, or build a variable with `MaxPathLength = 0` of type `PkiIssuerSignIntermediateRequest` and `json.Encode` it. You will notice that json will not contain this value.

# Additional Information

I think this property type should be a pointer (`*int32`). (Or `omitempty` removed completely).

Start with schema/model_pki_issuer_sign_intermediate_request.go and inspect the MaxPathLength JSON field used by client.Secrets.PkiIssuerSignIntermediate. Reproduce the issue by encoding a request with MaxPathLength set to 0, then verify the request includes max_path_length and that the generated certificate has pathlen:0.

Written by the indexing model from the issue text.