Update API gateway custom domain names

[FreddieCarre]

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Feature Request

Is your feature request related to a problem? Please describe.
I need to update the certificate used by the API gateway created by serverless-domain-manager. As the old certificate references a domain we no longer own and so cannot auto-renew the cert over DNS.

Describe the solution you'd like
If a custom domain already exists, use the updateDomainName sdk call to apply the config to the existing domain name. No change to the serverless template is necessary, although could add a flag so users can disable this behaviour if they're happy with the current implementation. May be better to do this as an 'opt-in' so that there is no change for your current users who do not need to update in place.

custom:
  customDomain:
    domainName: foo.example.tld
    certificateArn: arn:......
    skipUpdate: false

Describe alternatives you've considered
Writing our own sls hook to create / update domain names on api gateway

Any Other Info
If you can, maybe sketch out implementation details? If you've started a PR for this feature, link it here.
Happy to raise PR as I made some changes to the local node_module code and managed to update our custom domain name successfully.

[FreddieCarre]

I have a branch with the initial changes, but do not have permissions to push to the repo 🤔 let me know if I can share