feat: Re-issue/send PDAs when first-party endpoint certificates are renewed

[gnarea]

Fixes #185. It's also worth highlighting the following:

• It introduces the instance method FirstPartyEndpoint.authorizeIndefinitely(), which is like FirstPartyEndpoint.issueAuthorization(), except that it tracks which 3rd party endpoints are authorised in order to renew their PDAs automatically.
  • For example, Awala Ping will continue to use FirstPartyEndpoint.issueAuthorization() but Letro will use FirstPartyEndpoint.authorizeIndefinitely().
  • To stop renewing an indefinite PDA, it's enough to delete the respective 1st or 3rd party endpoint.
• This PR introduces the concept of "channel" (e.g., ChannelManager), which exists in the Awala protocol suite and refers to an established communication channel between two endpoints (which is end-to-end encrypted and where both are mutually authorised to send messages to each other).
• Because #175 isn't implemented yet, I haven't actually integrated the function to reissue+send PDAs (FirstPartyEndpoint.reissuePDAs()). This should be done as part of #175.
• This PR integrates CertificationPath from the core Awala lib, which effectively duplicates the existing AuthorizationBundle data class. Once this PR is merged, I'll create a separate PR to replace the remaining uses of AuthorizationBundle.

Review notes

Of things that I'm likely to have got wrong here, I'd rank highly the use of SharedPreferences and coroutines (esp. where I have to specify a context explicitly), so please pay special attention to those.

[gnarea]

Hmm, one thing I just realised is that FirstPartyEndpoint.authorizeIndefinitely() and FirstPartyEndpoint.issueAuthorization() must actually return ByteArrays -- otherwise we'd be leaking the Awala lib to the apps using this lib.

[github-actions[bot]]

:tada: This PR is included in version 1.8.0 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket: