[Agent][Platform] Introduce Prompt Templates

[chr-hertel]

Feature

It is a common and simple feature to provide and use prompt templates:

• https://python.langchain.com/docs/concepts/prompt_templates/
• https://docs.llamaindex.ai/en/stable/examples/prompts/rich_prompt_template_features/
• https://learn.microsoft.com/en-us/semantic-kernel/concepts/prompts/prompt-template-syntax

Those linked solutions are quite similar, at least at first glance, and I guess we could see some use-case here already with the SimilaritySearch tool:

ai/src/agent/src/Toolbox/Tool/SimilaritySearch.php

Lines 50 to 53 in dd03c20

	$result = 'Found documents with following information:'.\PHP_EOL;
	foreach ($this->usedDocuments as $document) {
	$result .= json_encode($document->metadata);
	}

Open Questions

• I feel like this should live in the Platform component and sit on top of a message, like a TemplateMessage or something - not sure tho
• Should we go all in with twig here? feels like over the top 🤔