[Agent] Add basic setup for memory injections to system prompt

[DZunke]

Q	A
Bug fix?	no
New feature?	yes
Docs?	yes
Issues
License	MIT

See original contribution at php-llm/llm-chain#387

This PR introduces a flexible memory system that allows the LLM to recall contextual information that are permantent for the conversation. In difference to tools it can be always utilized, when use_memory option is not disabled. It would be possible to fetch memory by a tool with a system instruction like always call the tool foo_bar but, for me, this feels like a bad design to always force the model to do a tool call without further need.

Currently i have added just two memory providers to show what my idea is. I could also think about a write layer to fill a memory, together with a read layer, this could, for example, be working good with a graph database and tools for memory handling. But these are ideas for the future.

So for this first throw i hope you get what i was thinking about to reach. I decided to inject the memory to the system prompt, when it is available, instead of adding a second system prompt to the message bag. But this was just a 50/50 thinking. I tried both seem to be working equally, at least for open ai. I am open to change it back again.

What do you think?

[chr-hertel]

Thanks for moving this - that saves me some time 🙏

Found one small left over in the example, and please remove declare(strict_types=1); - then we're good to merge this :)

[OskarStark]

please remove declare(strict_types=1); - then we're good to merge this :)

Can we update the PHP-CS-Fixer config?

[chr-hertel]

Looks good now 👍

[chr-hertel]

Thank you @DZunke.