Add bindings for the pinned metrics

[aumetra]

Add Go FFI bindings for the new pinned metrics. Uses MessagePack as the encoding format for complex types across the boundary.

Follow-up to CosmWasm/cosmwasm#2034

(This is me whipping out my rusty (haha, funny pun) Go skills from around (if not before) Go 1.11)

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TODO:

• Tests on Go side

[aumetra]

This PR also adds a Nix flake. I can remove it again if it's not wanted in-tree, I just used it to set up a Go development environment quickly. Installing everything manually would have taken me longer, so a few lines of Nix and a nix develop later made that way quicker

[webmaster128]

Very nice. Just the encoding question and some minor ideas here

[webmaster128]

Looks good. Let's wait for a 2.1 pre-release tag before we merge this

[chipshort]

The 2.1.0-rc.1 is released now and merged on main, so this can be updated to that.

[aumetra]

@chipshort Rebased on main

[webmaster128]

Nice. I just don't recall anymore what the reason behind the comment was

[aumetra]

Okay, I finally figured it out. I really don't know Go well enough..
Basically, our previous MessagePack library in Go didn't like the new format. But doesn't matter because this new library doesn't care and is actually faster.

[chipshort]

Code looks good, just wondering about the flake

[chipshort]

LGTM