Support operating without upstream URL

[alukach]

If someone runs the stac auth proxy in a non-proxy mode (e.g. using configure_app()), we should allow the user to omit the upstream URL.

This would require updating the following middleware and handler:

stac-auth-proxy/src/stac_auth_proxy/app.py

Lines 139 to 144 in 40444cf

	if settings.root_path or settings.upstream_url.path != "/":
	app.add_middleware(
	ProcessLinksMiddleware,
	upstream_url=str(settings.upstream_url),
	root_path=settings.root_path,
	)

stac-auth-proxy/src/stac_auth_proxy/app.py

Lines 79 to 83 in 40444cf

	if settings.healthz_prefix:
	app.include_router(
	HealthzHandler(upstream_url=str(settings.upstream_url)).router,
	prefix=settings.healthz_prefix,
	)