kad: Providers part 7: better types and public API, public addresses & known providers

[dmitry-markin]

This PR introduces the following changes:

1. Introduces a better type ContentProvider without a burden of KademliaPeer.
2. Simplifies public litep2p content providers API.
3. Uses PublicAddresses API when advertising local providers.
4. Adds locally known providers to discovered providers when performing GET_PROVIDERS request.
5. Emits IncomingProvider event when remote node registers as a provider via ADD_PROVIDER request.

It may be easier to review this PR through individual commits. The commits of interest are:

1. Introduce ContentProvider type
2. Use PublicAddresses API
3. Use locally known providers when performing `KademliaHandle::get_providers() request
4. Emit IncomingProvider event

All other commits are merge commits from upstream branches.

Builds upon #245.

[lexnv]

LGTM!

Will probably need to take a quick look at the first PR in the series after all others are merged into it 🙏

[dmitry-markin]

Will probably need to take a quick look at the first PR in the series after all others are merged into it 🙏

My idea was to merge PRs one-by-one into master. When the first PR is merged, GitHub will automatically set the target branch for the second PR to master, and so on.

[lexnv]

Sounds like a plan! 👍