pass `membership` to `is_valid_cert`

[tbro]

See coment #3867 (comment) and zullip discussion.

• is_valid_cert should take a type implementing Membershipas input
• the is_valid_cert impl should directly call the threshold/stake table it needs from Membership (stake_table/success, stake_table/upgrade or da_stake_table/da_success). I think CERT::stake_table and CERT::threshold might be available for this, but maybe these should be on Voteable? haven't fully thought through this
• the build_cert function in testing looks fine, but worth a double-check
  I might be mixing some things up here, but basically we want to make sure anything directly taking a stake_table or threshold is refactored to only take a Membership, where possible. I think this should be a fairly small refactor but should add a lot of safety

[sveitser]

Should be done after #3966