Auto-join support for IPv6 discovery

[dekimsey]

The go-discover library returns IP addresses and not URLs. It just so
happens net.URL parses "127.0.0.1", which isn't a valid URL.

Instead, we construct the URL ourselves. Being careful to check if it's
an ipv6 address and making sure it's in explicit form if so.

Fixes #12323

[pmmukh]

The PR looks pretty great! One thing, could you maybe try to add a test similar to this https://github.com/hashicorp/vault/blob/main/vault/external_tests/raft/raft_test.go#L129, that does the same but for ipv6 addresses ? It's probably gonna be fairly tricky, I can't exactly tell how much, but a test to show this change works would be nice i think.

[dekimsey]

I would be happy to try. I wanted to add a test when I wrote the PR up, but I couldn't figure out how to do that. I didn't see this area of the codebase though, so I'll give it a whirl!

[pmmukh]

sounds great, and thanks!

[ncabatoff]

The test TestRaft_RetryAutoJoin is of questionable value, because it doesn't actually exercise autojoin. In fact, if you remove the AutoJoin field from the LeaderJoinInfo struct, or mangle it to be invalid go-discover syntax, the test doesn't fail, so I'm kind of inclined to remove it if we can't improve it.

Sadly we don't currently have a good way of running our automated CI tests with access to AWS, so it may well be that there's no good way to test this change other than manually.

[pmmukh]

aha gotcha, thanks for the clarification @ncabatoff ! In that case, please feel free to ignore the request for a test addition @dekimsey , once I've validated the fix works on my end, will merge this in.