feat: export NewKerberosClientFunc to config for allow custom client

[fooofei]

what this pr is

The new pr is a custom config enhancement that allows the creation of user-defined KerberosClient.

After allow custom NewKerberosClientFunc, then we can custom spn, this also will satisfy https://github.com/IBM/sarama/pull/2437/files

how to implement

move NewKerberosClientFunc from GSSAPIKerberosAuth private field to GSSAPIConfig public field, allow user set NewKerberosClientFunc from config struct.

tests result

?       github.com/IBM/sarama/internal/toxiproxy        [no test files]
?       github.com/IBM/sarama/tools/kafka-console-consumer      [no test files]
?       github.com/IBM/sarama/tools/kafka-console-partitionconsumer     [no test files]
?       github.com/IBM/sarama/tools/kafka-console-producer      [no test files]
?       github.com/IBM/sarama/tools/kafka-producer-performance  [no test files]
?       github.com/IBM/sarama/tools/tls [no test files]
ok      github.com/IBM/sarama   31.257s
ok      github.com/IBM/sarama/mocks     (cached)

[fooofei]

@dnwe @hindessm can you accept this pr ？
this pr is simple
someone can accept this ?

[dnwe]

@fooofei whilst the change looks simple on paper, it's technically a change to public facing APIs:

• GSSAPIConfig: old is comparable, new is not
• GSSAPIKerberosAuth.NewKerberosClientFunc: removed

So we need to have a think about whether anyone in reality could have been relying on the previous APIs and hence how we can best handle this change

[fooofei]

GSSAPIConfig comparable is meaning-less, no need to compare
GSSAPIKerberosAuth is used at type Broker struct as private field kerberosAuthenticator, so remove GSSAPIKerberosAuth.NewKerberosClientFunc is safe.

@dnwe do you have other better idea for allow user to create customed KerberosClient ?
or maybe can we accept this pr?

can you response ?
please response 11

@hindessm