feat: add attachments handler to the sidecar

[Jiloc]

Description

As detailed in stacks-core issue #5558, we discovered that when connected to a syncing stacks-node, the node rarely sends POST requests to the configured event observers at the /attachments/new endpoint - even if the events observer is configured to filter only specific events.

This issue was observed during a sync starting from the genesis block, though it is unclear whether it also occurs with more recent messages.

Changes

• added dummy handler for /attachments/new

Testing Information

• add unit tests to confirm that the enpoint always returns 200 whatever the input

Checklist:

• I have performed a self-review of my code
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes