feat: allow to create static instances of some objects

[flavio]

trafficstars

Allow to create static instances of the cosign::Client, sigstore::trust::ManualTrustRoot.

The ability to create static instances of these objects can simplify the downstream consumers of this library.

The approach taken to create these static instance is the same used by rustls_pki_types::CertificateDer::into_owned

[flavio]

Putting on hold, because of https://github.com/sigstore/sigstore-rs/pull/311. This is probably going to be closed

[flavio]

Closing, this is no longer needed with the latest changes that landed into the main branch