Steep doesn't seem to be a big fan of Structs so I just went ahead
and turned the `AgentSettings` into a regular class that's equivalent
to the struct we had before.

(In particular, I decided to still keep every field as optional).

Ideally this would be a `Data` class, but we're far from dropping
support for Rubies that don't have it.

This is preparation to also share this behavior with profiling.

The behavior from the old `AgentBaseUrl` is now contained in
`AgentSettings` so we can clean up the extra logic.

**What does this PR do?**

This PR builds atop #4237 and fixes a similar-ish issue in the profiler
caused by the same mishandling of ipv6 addresses.

In particular, when provided with an ipv6 address in the agent url,
the profiler would fail with an exception:

```
$ env DD_AGENT_HOST=2001:db8:1::2 DD_PROFILING_ENABLED=true \
bundle exec ddprofrb exec ruby -e "sleep 2"

dd-trace-rb/lib/datadog/profiling/http_transport.rb:27:in `initialize':
Failed to initialize transport: invalid authority (ArgumentError)
```

**Motivation:**

Luckily we didn't have any customers using this, as it fails immediately
and loudly, but it's still a bug on a configuration that should be
supported.

**Additional Notes:**

Since we had similar buggy logic copy-pasted in crashtracking and
profiling (crashtracking had been fixed in #4237) I chose to extract
out the relevant logic into the `AgentSettings` class, so that
both can reuse it.

**How to test the change?**

I've added unit test coverage for this issue to profiling, and
the snippet above can be used to end-to-end test it's working fine.

Here's how it looks on my machine now:

```
E, [2025-01-02T17:32:32.398756 #359317] ERROR -- datadog: [datadog]
(dd-trace-rb/lib/datadog/profiling/http_transport.rb:68:in `export')
Failed to report profiling data (agent: http://[2001:db8:1::2]:8126/):
failed ddog_prof_Exporter_send: error trying to connect: tcp connect
error: Network is unreachable (os error 101): tcp connect error:
Network is unreachable (os error 101): Network is unreachable (os error 101)
```

E.g. we correctly try to connect to the dummy address, and fail :)

(Note: The error message is a bit ugly AND repeats itself a bit.
That's being tracked separately
in DataDog/libdatadog#283 )

Forgot this one, some of our tests relied on it!