feat(tls): add SANs support for both DNS-names and URIs

[lucab]

This adds two new Lua functions (and the corresponding C logic) to configure custom SANs validation in nginx:

• resty.kong.tls.set_upstream_ssl_sans_dnsnames()
• resty.kong.tls.set_upstream_ssl_sans_uris()

Part of KAG-6247.

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Notes:

• This requires a corresponding nginx patch implementing the additional TLS-name validation logic.
• Testing for this change (and the above nginx patch) is done at the integration level in Kong, see linked PR.

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[bungle]

@lucab, very good. I approve.

[samugi]

lgtm. Is this testable? Should we have something in t/?

[lucab]

Is this testable? Should we have something in t/?

I think this is not testable in isolation (at least not in a meaningful way), as it works in tandem with the related nginx patch wrt. TLS-name validation logic.
However, the composite result of those patches is covered by a new integration test in Kong.

[lucab]

@bungle @samugi Thanks for the reviews! Two additional notes on this:

• I just realized the new functions should be documented in the README as well. I added a commit on top with doc entries only.
• Even if green, I can't self-merge in this repository. Can you please merge this for me?