Multisig upgrade support

[hiddentao]

Multisig upgrade support should be added to enable protected upgrades.

A possible proposal:

• Built-in n-of-m multisig system in the Diamond (replaces the Ownership facet).
• Initially it's a 1-of-1, so it works the same as current Diamond deployments.
• Allow for batch consensus (i.e. signers have created off-chain signatures which then get sent in together).
• Permission logic should be pluggable, e.g if you want certain signers to always be a signatory

Alternative proposal:

• Use existing solutions such as Gnosis Safe as the owners of the diamond and build tooling that integrates with them.