Implement id_graph_stats getter

[Kailai-Wang]

Context

Some tech-debt from old TODO:

litentry-parachain/tee-worker/app-libs/stf/src/getter.rs

Lines 198 to 204 in 585f332

	// TODO: we need to re-think it
	// currently, _who is ignored meaning it's actually not a "trusted" getter.
	// In fact, in the production no one should have access to the concrete identities
	// but maybe it makes sense to get some statistic information
	// Disabled until it's resolved
	// Disabled the test `lit-id-graph-stats` too
	TrustedGetter::id_graph_stats(_who) => None,

We need to differentiate between dev and prod implementations.

In production we only want some generic statistics, e.g. how many unique IDGraphs were created, the distribution of identities in each IDGraph etc.

In dev we could additionally list all IDGraphs.

It should not be a trusted getter (as it's not specific to an account which needs authentication).

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