Event dispatcher does not filter attachments when event_keys is configured

[cylewitruk]

Describe the bug
Not sure if it could be a bug or if it's expected, but @Jiloc reported that while trying to do a genesis sync on testnet with an event-observer configured using specific event_keys, he's received unexpected calls to /attachments/new, which isn't included in his event observer config:

[[events_observer]]
endpoint = "sidecar:5000"
events_keys = [
    "SN2ZNBHMSQDZCAFWTX2KYDSXG0TGV1MAJNZV2HSHS.sbtc-registry::print",
]

Since these events aren't handled by his observer, they return 404 and thus block the node from progressing.

@obycode had a quick look and his analysis was:

It looks like the event dispatcher does not filter the list of registered observers at all for attachments. I don't know if that is intentional or not.

Steps To Reproduce
Performing a genesis sync with an event observer configured which does not handle the /attachments/* events should reproduce this.

Expected behavior
As a user, if I supply specific event_keys, I would not expect to receive other, non-related events.

Environment (please complete the following information):

• OS: Ubuntu
• 3.1.0.0.0 (docker release image)

[rdeioris]

Confirmed:

[[events_observer]]
endpoint = "localhost:3700"
events_keys = ["memtx"]

will send any events.

Working on the fix right now.