[performance] reduce InboxForward->Create calls by partially implementing Exists()

[NyaaaWhatsUpDoc]

Description

This adds a simple in-memory map that stores the IDs of activities that we have recently created, which allows us to partially implement Exists() and reduce the number of unnecessary calls to Create() during inbox forwarding. Unfortunately we can never return a definite answer in Exists(), since the activity JSONLD-ID doesn't necessarily match the stored object's JSONLD-ID, which is the only part that we actually store in a lot of cases. So this provides a best-effort implementation to at least reduce unnecessary calls for recently handled activities.

In the future, I don't think we should strive to "fully" implement Exists(), instead focusing on removing our reliance on the go-fed code for federation logic.

Partially handles #1891

Checklist

• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have not leveraged AI to create the proposed changes.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[tsmethurst]

Nice little optimization! :)