Swift 6

[btoms20]

What

• Bumps spm to 6.0 (libp2p will start supporting the latest 3 versions of swift similar to swift-nio)
• Migrates from XCTest to Testing
• Adds Sendable conformance to necessary types

Breaking Changes

Warning

• Many of the protocol signatures have changed due to Sendable requirements
• This PR requires a minor version bump and depends on the +0.2.0 series of support dependencies