Add validation for secret used with the Splunk output in CLF

Description of problem:

If the hecToken secret key is missing or misspelled for the Splunk output, Vector sends HTTP requests to Splunk without the token in the authorization header

authorization: Splunk <blank>

that rightfully get rejected.

Version-Release number of selected component (if applicable):

5.8, 5.7.z

How reproducible:

Always

Steps to Reproduce:

1. Create a secret for the Splunk output, misspell the name of the hecToken key
2. Create a CLF with a Splunk output, with the secret created in the previous step
3. Instantiate the CLF

Actual results:

Vector forwards logs to Splunk without a token in the authorization header, gets rejected

Expected results:

CLO fails CLF validation, sets the status with a message about missing hecToken key