[Downstream Usability] Autobuild RPM packages on every merged commit

This is a not a request for improvement in wasmtime itself, but rather in downstream ease-of-use which will need some upstream changes. I will submit a PR for this if upstream is cool with accepting it :crossed_fingers:

#### Feature

Enable automatic rpm package builds on every upstream PR merge.

#### Benefit

1. Users can conveniently use the latest features available upstream
2. No need to wait for upstream to cut a new release
3. Near-immediate availability to distro users
4. Delivered way quicker compared to official distro packages

#### Implementation

This will involve:
1. adding a `.copr/Makefile` and a `.copr/wasmtime.spec` such that:
`make -f .copr/Makefile srpm` generates an srpm file which the copr infrastructure can use to build rpms on their build infra.

An example can be found for [crun](https://github.com/containers/crun/tree/main/rpm) along with a [successful rpm build](https://copr.fedorainfracloud.org/coprs/rhcontainerbot/podman-next/build/4692536/).

We also build our [other packages](https://copr.fedorainfracloud.org/coprs/rhcontainerbot/podman-next/packages/) including Podman, Buildah and Skopeo similarly.

2. adding a webhook to the wasmtime repo to trigger builds on the copr after every PR merge.

#### Alternatives

1. Get users to build from source
2. Wait for distro package to ship required features
3. I could do copr integration from a fork which would have to be perpetually rebased (I really hope not :sweat: :smile: )

/cc @giuseppe @font @flouthoc