fix: default api host when omitted

[marandaneto]

💡 Motivation and Context

api_host had a default value internally, but the config schema still required users to provide it. This makes api_host optional so SDK initialization can use the default PostHog host when omitted.

💚 How did you test it?

• mix test test/posthog/config_test.exs

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran sampo add to generate a changeset file
• Added the release label to the PR