feat: add support for custom domain proxy

[ngarg-mparticle]

Summary

• Read the connection setting "customDomainProxyUrl" and set it

Testing Plan

• Was this tested locally? If not, explain why.
• Tested without setting customDomainProxyUrl in mP UI first. The integration worked normally and events were forwarded to Airship.
• After setting a dummy value "https://analytics.example.com" for customDomainProxyUrl in mP UI, the integration stopped working and I started getting "A server with the specified hostname could not be found." in the Xcode console.
• Further testing needs to be done with a working custom domain proxy.

[BrandonStalnaker]

@ngarg-mparticle any update on testing this?

[ShaneQi]

Why are we setting URLAllowList to UAConfigCustomDomainProxyUrl? I assume this would overwrite the URLAllowList populated from the default config, which is probably not what we want to do.

Context: I'm from the customer that requested this feature. I will be testing this and the above doesn't seems to be right.

[ngarg-mparticle]

@ShaneQi this change was made following Airship's docs here: https://docs.airship.com/reference/integration/custom-domain-proxy/?groupid=ios-swift

[ngarg-mparticle]

@ShaneQi perhaps a better approach would be to append if URLAllowList already exists? I can look into making that change soon.

addressed

[BrandonStalnaker]

LGTM