Custom Domain for API Portal

# What are you trying to achieve?

We would like to have a custom domain fro API Center Portal. Currently we need to get exception for the api center portal in zscalar to be able to access apis in it. If the customer created for api centers in future lets say 100, each time we would need to get the exception as by default the api portal is not showing the APIs. If custom domain is there this issue would be resolved.

# Proposed solution

A clear and concise description of what you want to happen.

# Alternative solutions

A clear and concise description of any alternative solutions or features you've considered.

# Additional context

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