Standardized SSL settings

[edmocosta]

• Deprecated ssl_enable in favor of ssl_enabled
• Deprecated ssl_cert in favor of ssl_certificate
• Deprecated ssl_verify in favor of ssl_client_authentication when mode is server
• Deprecated ssl_verify in favor of ssl_verification_mode when mode is client
• Added ssl_cipher_suites configuration and common validations
• Fixed the server mode when SSL is enabled

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Closes elastic/logstash#14926

[kaisecheng]

Docs and the actual change look great. I have tested the settings manually with self-signed cert with tcp-input.

For the docs, do you think we can add a table or example config to demonstrate the available settings for server mode and client mode? When I read from the beginning, It is not easy to spot that server mode uses ssl_client_authentication, while client mode uses ssl_verification_mode.

For the test, do we have test cover mTLS with ssl_certificate_authorities? It will be great to have it to get more confident to this plugin

[edmocosta]

Thanks for reviewing @kaisecheng! :)

This plugin has almost no test coverage for the server mode and for the already existing SSL settings. I've added a bunch of them but couldn't cover it all on this PR, which main focus was standardizing the existing settings.
Adding full coverage + integration tests would take a considerable amount of time, which, IMO, should be spent on this unifying issue.

I've added new tests to ensure the OpenSSL cert_store is being configured properly, it's the minimum requirement to make ssl_certificate_authorities work on both modes.

Regarding the docs, the only settings that are specific per mode are the ssl_client_authentication and ssl_verification_mode, all the other settings are shared between them. I can definitely add a table if you think it's necessary, but IMO, the note on those settings description should be enough to identify in which mode it should be used. (I've fixed the ssl_verification_mode one description, it was missing the NOTE formatter)

[kaisecheng]

agree we can cover the tests in another PR and not block this feature.
Awesome work for standardisation. LGTM