Support transient identities and traits in Core API

[khvn26]

Part of #4278.

Evolve the API schemas as per the following:

GET /api/v1/identities (docs):

{
    "get": {
        "operationId": "identify_user",
        "description": "",
        "parameters": [
            {
                "name": "identifier",
                "in": "query",
                "required": true,
                "type": "string",
                "minLength": 1
+            },
+            {
+                "name": "transient",
+                "in": "query",
+                "required": false,
+                "type": "boolean"
            }
        ],
        "responses": {
            "200": {
                "description": "",
                "schema": {
                    "$ref": "#/definitions/SDKIdentitiesResponse"
                }
            }
        },
        "tags": [
            "api"
        ]
    }

POST /api/v1/identities (docs):

#/definitions/IdentifyWithTraits

{
    "required": [
        "identifier"
    ],
    "type": "object",
    "properties": {
        "identifier": {
            "title": "Identifier",
            "type": "string",
            "minLength": 1
        },
        "traits": {
            "type": "array",
            "items": {
                "$ref": "#/definitions/TraitSerializerBasic"
            }
        },
        "flags": {
            "type": "array",
            "items": {
                "$ref": "#/definitions/SDKFeatureState"
            },
            "readOnly": true
+        },
+        "transient": {
+            "type": "boolean",
+            "x-nullable": true
        }
    }
}

#/definitions/TraitSerializerBasic

{
    "required": [
        "trait_key",
        "trait_value"
    ],
    "type": "object",
    "properties": {
        "id": {
            "title": "ID",
            "type": "integer",
            "readOnly": true
        },
        "trait_key": {
            "title": "Trait key",
            "type": "string",
            "maxLength": 200,
            "minLength": 1
        },
        "trait_value": {
            "title": "Trait value",
            "type": "string",
            "x-nullable": true
+        },
+        "transient": {
+            "type": "boolean",
+            "x-nullable": true
        }
    }
}

When endpoint gets called:

• If identity is marked as transient, return the evaluated flags, but don't persist it.
• If a trait is marked as transient, use it when evaluating flags, but don't persist it.