rekorv2: request timeouts

[ramonpetgrave64]

Client support for Rekor V2: sigstore-python #289

Description

Currently the RekorV2 client has no timeout for requests, which means that requests may take forever.
We should have some sensible timeout for the client, at least 20 seconds, but probably no longer than 60 seconds.

• https://github.com/sigstore/rekor-tiles/blob/28f3c79d9855192fde1e896c27c4b7b225c7436f/CLIENTS.md#handling-longer-requests