[Filebeat] Add base64 function to template engine

[andrewkroh]

Describe the enhancement:

Add a base64 function to the template engine for httpjson.

Describe a specific use case for the enhancement or feature:

To be able to base64 encoding a string that is added to a header. Specifically this base64 will be used encode part of an Authorization header.

- type: httpjson
  request.transforms:
    - set:
        target: header.Date
        value: '[[ formatDate (now) ""Mon, 02 Jan 2006 15:04:05 -0700"" ]]'
    - set:
        target: header.Authorization
        value: 'Basic [[ base64 (printf "%s:%s" .IntegrationKey (hmac "sha1" .SecretKey .header.Date "\n" .request.Method "\n" .url.Host "\n" .url.Path "\n" .url.Query)) ]]'

In this example I doubt the keys I referenced in the HMAC are correct for the httpjson input, but those are the pieces of data that need to be include in the hash.

https://duo.com/docs/adminapi#authentication describes the format of the authorization header above and provides a python example.

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[P1llus]

I will work on this together with Sai