[1st PR of Certificate Selector] Support parsing Kubernetes secrets containing intermediate certificates

[Xulei-NL]

Background

I tried to create a PR for my new Certificate Selector logic. However the code without test cases exceeds 600 lines, which was too large for code review.

Plan

This is the plan to split up the whole Certificate Selector logic into 6 small PRS.

Order	PR feature	Changes	Focused tests
1	Parse TLS Secrets and certificate chains	TlsSecretCertificateParser and its interface	Update TlsSecretCertificateParserTests to assert the returned leaf and intermediates individually. Cover PEM and DER input, missing data, and certificate chains. Avoid checking the tuple itself for null: assert its elements.
2	Manage certificate storage and safe rotation	TlsSecretCertificateStore and its interface	Test acquire, replace, remove, lease release, and deferred disposal. Depends on PR 1's parsed leaf and chain.
3	Resolve Ingress TLS hosts to Secrets	TlsIngressBindingIndex and its interface	Test host and wildcard matching, collisions, replacement/removal, and synchronization.
4	Feed Ingress and Secret events into TLS state	TLS-related IngressCache changes and required DI wiring	Test cache updates, deletes, invalid Secrets, and controller-class filtering. Depends on PRs 2 and 3.
5	Select a certificate from SNI	Selector changes and the Kestrel TLS handshake callback, initially with a single HTTPS listener	Test matching certificates, default-certificate fallback, and missing certificates. Depends on PR 4 and the TLS store/index wiring.
6	Support multiple HTTPS listen ports	HttpsListenPorts and multi-port Kestrel configuration	Test that each configured port is bound and uses the selector. Keep this separate from the initial SNI implementation.

This PR

This PR is the first one to support parsing k8s certificate secrets without breaking any existing code.

Feedback appreciated

I'm appreciated to all feedbacks.

[Xulei-NL]

Have no idea why the test pipeline for macOS failed almost every time. 😟 Couldn't see the test details.