Add support for custom JSON Schema instance formats defined by users

[Fatal1ty]

This pull request updates the type annotation for the format field in the JSONSchema dataclass. The annotation is changed from JSONSchemaStringFormat | JSONSchemaInstanceFormatExtension to JSONSchemaInstanceFormat. This modification enables users to implement and utilize custom classes that inherit from JSONSchemaInstanceFormat.

The functionality provided by the pull request #232 can now be achieved using a plugin:

from typing import Annotated

from mashumaro.jsonschema import build_json_schema
from mashumaro.jsonschema.models import (
    Context,
    JSONSchema,
    JSONSchemaInstanceFormat,
)
from mashumaro.jsonschema.plugins import BasePlugin
from mashumaro.jsonschema.schema import Instance


class CustomJSONSchemaInstanceFormatPlugin(BasePlugin):
    def get_schema(
        self,
        instance: Instance,
        ctx: Context,
        schema: JSONSchema | None = None,
    ) -> JSONSchema | None:
        for annotation in instance.annotations:
            if isinstance(annotation, JSONSchemaInstanceFormat):
                schema.format = annotation
        return schema


class Custom1InstanceFormat(JSONSchemaInstanceFormat):
    CUSTOM1 = "custom1"


assert build_json_schema(
    Annotated[str, Custom1InstanceFormat.CUSTOM1],
    plugins=[CustomJSONSchemaInstanceFormatPlugin()],
).format is Custom1InstanceFormat.CUSTOM1