Allow to configure additional Scrapy settings when using savefixture command

[VMRuiz]

During the execution of savefixture command there are several Scrapy settings configure ad-hoc to be able to generate the tests:

scrapy-poet/scrapy_poet/commands.py

Lines 122 to 130 in 33884cf

	self.settings["DOWNLOADER_MIDDLEWARES"][
	"scrapy_poet.InjectionMiddleware"
	] = None
	self.settings["DOWNLOADER_MIDDLEWARES"][
	"scrapy_poet.downloadermiddlewares.InjectionMiddleware"
	] = None
	self.settings["DOWNLOADER_MIDDLEWARES"][InjectionMiddleware] = None
	self.settings["DOWNLOADER_MIDDLEWARES"][SavingInjectionMiddleware] = 543
	self.settings["_SCRAPY_POET_SAVEFIXTURE"] = True

It would be great if we could pass additional ones, for example, to configure ZAPI without the need for an underlying Scrapy project. Such configuration could be defined in pyproject.toml or passed ass cli args.