experimentalHttps: Use custom experimentalHttpsCa without Key and Cert

[ericponto]

Goals

1. Allow the use of a specific CA file, while still having Next.js create the key and cert.

Non-Goals

No response

Background

When using experimentalHttps, Next is checking for the existence of both the experimentalHttpsKey and experimentalHttpsCert option. If they both exist, then it creates the certificate using those file paths, plus the experimentalHttpsCa as the rootCa. If one or both doesn't exist, it creates a certificate (with createSelfSignedCertificate) which creates the key, cert, and rootCa.

At my company, we already have a root CA, and process.env.NODE_EXTRA_CA_CERTS is already set using it. We need this to get through the corporate proxy/firewall.

Proposal

Separate out the rootCa form the key and cert check and allow experimentalHttpsCa to be passed in on its own. Then use experimentalHttpsCa, even when generating the key and cert.