Move HTTP security to GUCs instead of Cargo features.

[thomcc]

Currently, the HTTP security level is controlled by three cargo features (

pg_durable/Cargo.toml

Lines 18 to 20 in 69eefdc

	http-allow-all = []
	http-allow-azure-domains = []
	http-allow-test-domains = ["http-allow-azure-domains"]

), which must be enabled or disabled at the time when the extension is built. Ideally, this would be done via one or more Postmaster GUCs (probably SUPERUSER_ONLY), so that it can be changed more flexibly.