[FR] Add WebApps support to project_management

从 project_management 包开始，将现有的 iOSApps 和 AndroidApps API 与拟议的 WebAppsService 进行比较。检查 Firebase REST 的 webApps 资源，确定除 metadata 和 config 之外的完整 API 范围；当 WebApps 支持与现有平台服务保持一致地对外提供，并由相关测试覆盖时，即视为完成。

Is your feature request related to a problem? Please describe.
The project_management package miss API to work with webApps resource

Describe the solution you'd like
API to work with WebApps similar to the ones available for iOSApps and AndroidApps

Describe alternatives you've considered
The single alternative I considered was to use the REST requests by hands

Additional context
I implemented a bare minimum solution just extending _ProjectManagementService

from firebase_admin import project_management


class WebApp:
    def __init__(self, app_id, service) -> None:
        self._app_id = app_id
        self._service = service

    @property
    def app_id(self):
        return self._app_id

    def get_metadata(self):
        return self._service.get_web_app_metadata(self._app_id)

    def get_config(self):
        return self._service.get_web_app_config(self._app_id)


class WebAppsService(project_management._ProjectManagementService):
    WEB_APPS_RESOURCE_NAME = 'webApps'

    def __init__(self, app):
        super().__init__(app)

    def list_web_apps(self) -> list[WebApp]:
        return self._list_apps(
            platform_resource_name=WebAppsService.WEB_APPS_RESOURCE_NAME,
            app_class=WebApp
        )

    def get_web_app_metadata(self, app_id):
        path = '/v1beta1/projects/-/{0}/{1}'.format(
            WebAppsService.WEB_APPS_RESOURCE_NAME,
            app_id,
        )
        return self._make_request('get', path)

    def get_web_app_config(self, app_id):
        path = '/v1beta1/projects/-/{0}/{1}/config'.format(
            WebAppsService.WEB_APPS_RESOURCE_NAME,
            app_id,
        )
        return self._make_request('get', path)

but it can be completed with all APIs