Add a supported hook for overriding the registry URL without writing config.yml

[drernie]

Problem

quilt3.session.get_registry_url() reads the registry URL from ~/.quilt/config.yml on every call (via get_from_config('registryUrl') in util.py). For embedding scenarios where a host application needs to drive multiple catalogs in one process (or simply override the URL without persisting it), the only options today are:

1. Call quilt3.config(url, ...) — which writes the user's config.yml on disk. Destructive for an embedder.
2. Monkey-patch quilt3.session.get_registry_url — which is what we ended up doing in quiltx, and it's a smell: process-global mutation of an imported module's internals, no clean lifetime, embedder-hostile.

Proposal

Add a supported, in-process override for the registry URL. A few possible shapes, in order of preference:

(a) A resolver hook

quilt3.session.set_registry_url_resolver(callable_or_None)

When set, get_registry_url() calls the resolver instead of reading config. None restores default behavior. Pairs naturally with a contextmanager helper.

(b) An explicit override + contextmanager

with quilt3.session.use_registry_url("https://catalog.example.com"):
    quilt3.admin.users.list()

(c) A registry_url= kwarg on the public entrypoints quiltx uses (admin modules, search, GraphQL client, login). More invasive but most explicit.

Any of these would let quiltx delete its monkey-patch + ContextVar machinery (~40 lines of quilt3_facade.py) and stop reaching into quilt3.session internals.

Why this matters

• quiltx advertises a Catalog.from_dns(...) embedding API. Without a supported hook, embedders inherit a process-wide quilt3.session.get_registry_url override that outlives their Catalog instance.
• Internal-fan-out check: get_registry_url() is currently called from session.py auth flows, search_util.py, and _graphql_client/base_client.py (which captures the URL at client construction). All synchronous within a single call stack — no async/thread fan-out — so any of (a)/(b)/(c) is safe to implement.

Happy to send a PR for whichever shape is preferred.