support multiple sigstore keys

[msmeissn]

Feature request description

At SUSE we currently want to transition the sigstore signing key from a RSA 2048 to 4096 bit key. During the transition time, there might exist containers signed with either of the keys. Currently, podman seems to be able to only configure 1 key for sigstore signing, not multiple ones.

Suggest potential solution

Support multiple sigstore keys, or perhaps reading from an index directory.

Have you considered any alternatives?

A clear and concise description of any alternative solutions or features you've considered.

Additional context

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