fast imports: [maybe] put progress callback url into spec on s3

[NanoBjorn]

There is import_pgdata_upcall_api in pageserver config and points to cplane where to send import progress when pageservers are done. It is ok for most environments to have single url per pageserver, apart from local setup, where we share pageservers between docker cplane and k8s cplane regions.

I understand that it might be too narrow use-case, but maybe we can consider putting callback url into the spec? This way pageserver would always know where to call about progress from the spec itself and it would be convenient for all kinds of setups. Maybe, it is also worth keeping config option as a default, but being able to override it from spec might be convenient.

Spec:

neon/pageserver/src/tenant/timeline/import_pgdata/importbucket_format.rs

Lines 17 to 20 in 86b9703

	pub struct Spec {
	pub project_id: String,
	pub branch_id: String,
	}

Smth like callback_progress_url makes sense, where we do POST as now.