feat: Add basic Multisig Account

[TAdev0]

Fixes #21

PR Checklist

• Tests
• Documentation
• Added entry to CHANGELOG.md
• Tried the feature on a public network

[TAdev0]

@martriay this is a first draft, only spent 2 hours on it.

Basically, i modified :

• get_public_key for get_public_keys , returning a span of all public keys stored associated with the account
• set_public_key for add_public_key and remove_public_key (with some checks missing for now)
• _is_valid_signature now makes sure that all provided sig are valid

It is just a draft so you can tell me if the way i implemented it is relevant or not.
Not that i added a threshold variable in storage, but did not implement for now a way to set it and modify it.

[martriay]

Thanks! Will look into it after we finish the current release.