Add Kafka `security.protocol` key to SpringBoot autoconfig

[Woodz]

Having some Kafka properties defined in SpringBoot autoconfig vs some Kafka properties having to be set in a separate "properties" map is not intuitive and is therefore confusing to junior devs. I propose that "security.protocol" is added to SpringBoot auto-configuration to avoid the separate "properties" key.

Spring Boot version: 2.1.9.RELEASE
Kafka version: 2.0.1

Example application.yml:

kafka:
  consumer:
    bootstrap-servers: <server>
    key-serializer: org.apache.kafka.common.serialization.StringSerializer
    value-serializer: org.apache.kafka.common.serialization.StringSerializer
    ssl:
      truststore-location: <trust store location>
      truststore-password: <password>
    properties:
      security.protocol: SSL

[wilkinsona]

This would be a step towards #17420 and is similar to #17389.

[mbhave]

@wilkinsona Can this be closed in favor of #17420?

[wilkinsona]

It could. However, given that #17420 is in the general backlog, I think we should consider treating it like #17389 and tackling it in 2.3.

[snicoll]

Closing in favour of PR #20360