There's a couple of idiomatic things in this sentence. There's "you're a good egg" to mean "you're a good person". Also, when a person has a mental breakdown, we say that the person has "cracked". So, "half-cracked" has a double meeting here that the person is "not perfect" because the egg shell is half-cracked, and also that he/she is a little "crazy".

It means that your friend likes you and thinks you are a good person, despite flaws.

'A good egg ' is an old fashioned expression for a good or kind person. 'Cracked' means you're crazy ( but in a good way, meaning you are silly or can make people laugh easily ) So the expression means good friends will love or like you because you are a good person, even if you act a little crazy at times