Fix malformed fallback streaming URL

[PattaFeuFeu]

Description

A user of the library mentioned in #530 that the streaming fallback URL is malformed in that it doesn’t contain ://, e.g. httpsmastodon.social instead of https://mastodon.social.

This PR ensures that the URL follows URL specifications by using HttpUrl to create a URL.
To not always run the builder, I’ve put it into a lambda that is only called if the fallback URL is actually necessary.

An easier way would be to just write $scheme://$instanceName but I decided against it so that OKHTTP HttpUrl is used on both ends, ensuring compatibility.

Closes #530

Type of Change

• Bug fix

Breaking Changes

• None

How Has This Been Tested?

Warning

Not yet tested. I’m currently in the process of writing unit tests for this.

Mandatory Checklist

• I ran gradle check and there were no errors reported
• I have performed a self-review of my code
• I have added tests that prove my fix is effective or that my feature works
• All tests pass locally with my changes
• I have added KDoc documentation to all public methods