Unmatched input / output of authgear config json

This is an issue discovered in https://github.com/authgear/authgear-server/issues/3106

## Description

Lets use the `tenant` field in [OAuthSSOProviderConfig](https://github.com/authgear/authgear-server/blob/2023C01/pkg/lib/config/identity.go#L393) as an example.

If we now update the config using the `updateApp` mutation, with the following input:

```jsonc
{
... // skip other configs because they are not important here
"providers: [{
... // skip other fields
"type": "azureadv2",
"tenant": ""
}]
}
```

The above input will pass the json schema validation, because `tenant` is not null.

Then, from the same graphql, we got the following output:

```jsonc
{
... // skip other configs because they are not important here
"providers: [{
... // skip other fields
"type": "azureadv2"
// <--- No "tenant" field
}]
}
```

The `tenant` field will be removed from the output, because empty string is the zero value of `string` in golang, and we've specified `omitempty` in the json.
And note that, the above output **will fail** the input validation because `tenant` is required, but was not specified.

## Solutions

1. Review all validation rules of json schema and add back necessary "minLength: 1" to strings.

2. Remove `omitempty` in required fields. Use pointer types `*string` / `*bool` if that field does not always exist.

Start with pkg/lib/config/identity.go, especially OAuthSSOProviderConfig and its tenant field, then trace the updateApp mutation's JSON schema validation and serialization. Compare required fields and omitempty behavior across the affected configuration types; done means valid updateApp input remains valid when returned by the same GraphQL API, with regression coverage for the round trip.

Written by the indexing model from the issue text.