feat: support generic http(s) urls

[coroa]

Fixes #5 .

With this change we re-introduce general http(s) urls based on a http head request, so that cached-http becomes a complete replacement for the original http plugin.

Of note: if head requests are unsupported, the plugin assumes the requested url exists with an mtime of 0.

[coroa]

We need to be a bit careful about releasing a version here, since i removed the monkey patching of snakemake-storage-plugin-http, so this would give us, if both are installled:

WorkflowError in file "/home/coroa/work/pypsa-eur/rules/retrieve.smk", line 28:
Multiple suitable storage providers found for query https://data.pypsa.org/workflows/eur/eurostat_balances/2026-02/nrg_bal_c: http, cached-http. Explicitly specify the storage provider.

I guess i could leave the monkey patching in place for a grace period and warn about the future incompatibility?

[lkstrp]

I mean this is currently used for PyPSA-Eur and forks only. We can just pin the current version in Eur, release here a new version, and remove the pin and remove the default storage provider

[coroa]

Yes, but since the update cycles of the soft-forks are somewhat irregular, let's just take the safe route and keep the monkey-patching around a bit longer.

[lkstrp]

Sorry for the missed review !