feat: add option for backend alias in chart

[samczsun]

I'm creating a deployment which uses an ALB Ingress (perhaps future PR to support different Ingress configurations) but for now the ability to configure the backend alias would be very helpful

[ikreymer]

Can you also add a commented out version of backend_alias to chart/values.yaml, explaining what it would do?
Not sure I fully understand the use case, is this an external alternative origin to access the frontend?
The APP_ORIGIN is used as a fallback when the origin can't be detected from the request..

[samczsun]

The APP_ORIGIN is used as the default_origin here

browsertrix/backend/btrixcloud/utils.py

Line 25 in f36fc91

default_origin = os.environ.get("APP_ORIGIN", "")

Which is returned as the origin if no headers are provided

browsertrix/backend/btrixcloud/utils.py

Lines 179 to 182 in f36fc91

	def get_origin(headers) -> str:
	"""Return origin of the received request"""
	if not headers:
	return default_origin

But no callers of get_crawl_out supply any headers, which means that my api was returning a backend URL of localhost:9870 for the pagesQueryUrl

browsertrix/backend/btrixcloud/basecrawls.py

Lines 158 to 165 in f36fc91

	async def get_crawl_out(
	self,
	crawlid: str,
	org: Optional[Organization] = None,
	type_: Optional[str] = None,
	skip_resources=False,
	headers: Optional[dict] = None,
	) -> CrawlOutWithResources: