changing type of SplunkToken from string to secret

[momoXD007]

What this PR does / why we need it:

In my last PR I added support for Splunk as output type. I however missed the fact that the so called SplunkToken is an authentication-token and should therefore be treated as a secret.

Context:
A SplunkToken is a Splunk generated string that can be used by programs to access a Splunk-instance's api. It is therefore a sensitive secret and should be handled as such.

Which issue(s) this PR fixes:

I think no one raised this externally, but the underlying issue that this fixes is that this change will force users to supply the SplunkToken as secret reference and this way prevents leaking of this secret.

Does this PR introduced a user-facing change?

action-required: if you used Splunk as an output-type and supplied the splunkToken as string you will need to change your code and are now forced to supply the splunkToken as a secret-reference. In the example below the splunkToken is loaded from a secret called splunk-token and within that secret it the token is stored under the key "myToken".

output:
      splunk:
        host: https://my.splunk.instance.net
        port: 8089
        eventIndex:  "Event"
        splunkToken:
          valueFrom:
              secretKeyRef:
                name: splunk-token
                key: myToken

Additional documentation, usage docs, etc.:

[benjaminhuo]

@momoXD007 Thanks!