Add template processor

[artemklevtsov]

Hi,

This PR adds a new template processor to Benthos that allows users to apply Go text/template templates to the structured content of messages. The processor supports optional Bloblang mapping for transforming data before templating, and can load templates from files using glob patterns.

This addresses the need for dynamic content generation using templating in Benthos pipelines, enabling use cases like generating dynamic JSON, XML, or text outputs based on message data.

Examples:

# example_1.yaml
input:
  generate:
    count: 1
    mapping: root.foo = "bar"

pipeline:
  processors:
    - template:
        code: "{{.foo}}"

output:
  stdout: {}

# example_2.yaml
input:
  generate:
    count: 1
    mapping: root.foo = "bar"

pipeline:
  processors:
    - template:
        code: "{{.value}}"
        mapping: "root.value = this.foo"

output:
  stdout: {}

# example_3.yaml
input:
  generate:
    count: 1
    mapping: root.foo = "bar"
  processors:
    - template:
        code: |
          {{ template "greeting" . }}
        files: ["./*.tmpl"]
output:
  stdout: {}

greeteing.tmpl:

{{- define "greeting" -}}
Hello, {{ .foo }}!
{{- end -}}

Output:

./target/benthos run example_3.yaml
INFO[2025-10-05T12:32:21+07:00] Running main config from specified file       @service=benthos benthos_version=ffd62dce19ed4f4211fa9e66c0600acbea59d569 path=tmp/glob.yaml
INFO[2025-10-05T12:32:21+07:00] Listening for HTTP requests at: http://0.0.0.0:4195  @service=benthos
INFO[2025-10-05T12:32:21+07:00] Input type generate is now active             @service=benthos label="" path=root.input
INFO[2025-10-05T12:32:21+07:00] Launching a Benthos instance, use CTRL+C to close  @service=benthos
INFO[2025-10-05T12:32:21+07:00] Output type stdout is now active              @service=benthos label="" path=root.output
Hello, bar!
INFO[2025-10-05T12:32:21+07:00] Pipeline has terminated. Shutting down the service  @service=benthos

Regards

[artemklevtsov]

@Jeffail, @mihaitodor could you give your feedback on the PR?

[Copilot]

Pull request overview

This PR introduces a new template processor for Benthos that enables users to apply Go text/template templates to structured message content. The processor supports optional Bloblang mapping for pre-templating data transformation and can load template definitions from files using glob patterns.

Key Changes

• New template processor with support for inline code and file-based template loading
• Integration with Bloblang for optional pre-processing of message data
• Configuration examples demonstrating basic templating, mapping integration, and file-based templates

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[artemklevtsov]

Looks like I merged the main upstream branch. My mistake.
I can send a new request if review of the current one is difficult.