[filebeat][httpjson] Add support for setting hash in request header.

[vinit-chauhan]

Describe the enhancement:
This issue here is to request support for adding the SHA256 hash of given strings and set it in the request header the current httpjson input of filebeat.

We need a function in the value template to generate the SHA256 digest of a given list of strings. Moreover, the function can be a generic hash function that can support different types of hashes for better compatibility with other APIs.

- type: httpjson
  request.transforms:
    - set:
        target: header.Authorization
        value: 'Basic [[ hash "sha256" "string-1" "string-2" ]]'

Describe a specific use case for the enhancement or feature:
Here's a reference link for generating a digest for AWS Signature Version 4,
https://docs.aws.amazon.com/general/latest/gr/sigv4-create-canonical-request.html

[elasticmachine]

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