Remove `async` from `sigstore::trust::TrustRoot` trait

[flavio]

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Description

Currently (v0.8.0) the [sigstore::trust::TrustRoot] trait has two methods, both of them async. These methods have been made async because of internal implementation of sigstore::trust::sigstore::SigstoreTrustRoot.

Internally SigstoreTrustRoot has to initialize its trusted_root variable, which involves the usage of futures. I think we can initialize this variable inside of the SigstoreTrustRoot::new method (which is already async) and simplify the trait definition.