Add NIP-45, which defines a COUNT verb

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+NIP-45
+======
+
+Event Counts
+--------------
+
+`draft` `optional` `author:staab`
+
+Relays may support the `COUNT` verb, which provide a mechanism for obtaining event counts.
+
+## Motivation
+
+Some queries a client may want to execute against connected relays are prohibitively expensive, for example, in order to retrieve follower counts for a given pubkey, a client must query all kind-3 events referring to a given pubkey and count them. The result may be cached, either by a client or by a separate indexing server as an alternative, but both options erode the decentralization of the network by creating a second-layer protocol on top of Nostr.
+
+## Filters and return values
+
+This NIP defines a verb called `COUNT`, which accepts a subscription id and a filter as specified in [NIP 01](01.md).
+
+Counts are returned using a `COUNT` response in the form `{count: <integer>}`. Relays may use probabilistic counts to reduce compute requirements.
+
+Examples:
+
+```
+# Followers count
+["COUNT", "", {kinds: [3], '#p': [<pubkey>]}]
+["COUNT", "", {count: 238}]
+
+# Count posts and reactions
+["COUNT", "", {kinds: [1, 7], authors: [<pubkey>]}]
+["COUNT", "", {count: 5}]
+```