feat: Add custom OIDC issuers to orgs

[miketheman]

Allows storing a specific issuer_url associated to an org, for later lookup.

Resolves #18831

[di]

LGTM aside from one nit/question!

[di]

A nit, maybe this should live in /warehouse/oidc/ instead? Also, should we find some way to ensure this remains consistent with the polymorphic_identity used for each of the individual publisher model types?

[miketheman]

I put it here since it's only currently used by this module, but will happily move it later if this approach works well.
I like the idea of constraining the potential values, but will defer for now, since it'll need to modify the mixin to be an enum choice, not a string, and update here:

warehouse/warehouse/oidc/models/_core.py

Lines 118 to 127 in 9cfbbf9

	class OIDCPublisherMixin:
	"""
	A mixin for common functionality between all OIDC publishers, including
	"pending" publishers that don't correspond to an extant project yet.
	"""
	# Each hierarchy of OIDC publishers (both `OIDCPublisher` and
	# `PendingOIDCPublisher`) use a `discriminator` column for model
	# polymorphism, but the two are not mutually polymorphic at the DB level.
	discriminator: Mapped[str | None] = mapped_column(String)