Make Integrations chainable

[rmeissner]

It should be possible to compose/ chain multiple plugins/hooks. This would allow for a better composability, i.e. it would be possible to reuse a time lock with different other plugins.

Open Questions:

• Should guards be applied to each component in the chain (or only last)?
• Should registry checks be performed for each component in the chain (or only last)?

[auryn-macmillan]

In Zodiac, we have a class of Modules called "Modifiers".
Modifiers expose IAvatar (the ModuleManager.sol interface) so that any modules built to call execTransactionFromModule() / execTransactionFromModuleReturnData() can target the Modifier, rather than the safe that will ultimately execute the call, in order to create a chain of mods.

Delay and Roles are the two canonical examples of Modifiers in Zodiac.

In Safe Protocol, we could achieve a similar outcome by having Modifiers expose ISafeProtocolMediator, so that other mods can target modifiers, rather than the mediator.