Feature: Support jsontemplate in http-bulk

[PranavSathy]

WARNING: AI Generated, utilized Codex 5.4 High Reasoning

Prompt:

This is a C++ repository for a plugin to "Halon" (https://halon.io) to support bulk sending HTTP requests
  to external systems while reading queued JSON objects located on Disk.

  The logic reads from `http-bulk.schema.json` in C++ (halon configuration) to construct the HTTP requests.

  Currently, the issue is that the only two formats that are really usable are `ndjson` and `jsonarray`.

  I want to introduce a new plugin configuration value (similar to format) called "jsontemplate" which supports
  formatting the JSON objects based on a template string before sending. My end goal (which needs to be unit testable) is generating the following HTTP request:

  {
      workflowType: { name: "handleHalonEmails" },
      taskQueue: { name: "lbx-live-halon-workflows", kind: "TASK_QUEUE_KIND_NORMAL" },
      identity: "halon-http-bulk",
      requestId: $UUIDV7,
      workflowTaskTimeout: "10s",
      input: {
          payloads: [
              {
                  metadata: { encoding: "anNvbi9wbGFpbg==" },
                  data: $DATA_BASE64
              }
          ]
      }
  }

  In this situation, I want to support a couple of custom tags:

  $UUIDV7 - A uuidv7 generator that subs out a UUIDv7 value within the template string.
  $DATA_BASE64 - The jsonarray into a base64 encoded json string.

This also wrote some extra tests to validate that this works.

NOTE: I am not C++ expert, I reviewed the code to the best of my ability, I suspect that this will result in some more mallocs considering the liberal use of std::string. I would encourage a more seasoned C++ reviewer take a look.

However, this totally works and offers some flexibility to the http-bulk plugin to interact with more modern systems like SQS, Temporal, etc. to enqueue tasks directly.