Add an option to enable self-service portal

[AustinZhu]

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Summary

Option to enable self-service portal in Client VPN module, alongside with an output of the URL of the portal.

Motivation

Self-service portal is commonly enabled in the setup of a client VPN endpoint.

Alternatives

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Additional Context

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Code of Conduct

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[sparkinson]

Hello,

This appears to be half done as there is an enable_self_service_portal input variable. But I think we also need one more variable for the "Self-service SAML provider ARN" and to add this to the authorization config when we have enable_self_service_portal set to true.

I'd be happy to help and create a PR with this change if that is useful

[adenot]

hi @sparkinson, we assumed it would use the saml_provider_arn for the self-service portal when enabled, if that's not the case, I'd appreciate if you send a PR with the changes.