BE: Support PEM trust/key stores and mTLS configuration. (#1437)

[iamtakingiteasy]

• [ ] Breaking change? -- Shouldn't be, configuration changes are append-only, unspecified store type is considered as JKS/PKCS12 (both can be handled under either and technically only a preference hint).

What changes did you make? Resolves #1437

Adds support for PEM keystore/truststore and mTLS configuration, replacing manual KeyManagerFactory/TrustManagerFactory calls with spring boot SslBundle to parametrize the kafka client and http/netty clients.

• TruststoreConfig/KeystoreConfig extended with truststoreType/keystoreType with possible values of JKS, PKCS12 and PEM
• KeystoreConfig is extended with keystoreCertificate for [pem] client certificate file.
• Cluster is extended with securityProtocol and kafkaSsl for mTLS client certificate.

Out-of-the scope FE change: ApplicationConfigPropertiesKafkaSchemaRegistrySsl -> KeystoreConfig as reusable DTO is introduced, auto-generated one no longer exists.

Is there anything you'd like reviewers to focus on?

How Has This Been Tested? (put an "x" (case-sensitive!) next to an item)

• [ ] No need to
• [x] Manually Tested on mTLS cluster with PEM, JKS and PKCS12 keystores
• [x] Unit checks
• [x] Integration checks
• [ ] Covered by existing automation

Checklist (put an "x" (case-sensitive!) next to all the items, otherwise the build will fail)

• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation (e.g. ENVIRONMENT VARIABLES) -- no environment variable changes is necessary in documentation compose files
• [x] My changes generate no new warnings (e.g. Sonar is happy)
• [x] I have added tests that prove my fix is effective or that my feature works -- a rather heavy setup would be required
• [x] New and existing unit tests pass locally with my changes
• [x] Any dependent changes have been merged -- no dependencies

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