ci: Add action to open cargo-release PR

[samuelburnham]

No description provided.

[huitseeker]

Missing pieces:

• please add a template for the release pr, so that reviewers know what they are looking at,
• one example: https://github.com/arkworks-rs/algebra/blob/master/.github/release-pr-template.ejs

Nice to have:

• it would be nice to do a minimum of checks on the version argument, if only to avoid typos.
  This snippet, which retrieves the current version, might help:

cargo metadata --format-version 1 | \
    jq --arg crate_name "neptune" --exit-status -r \
        '.packages[] | select(.name == neptune) | .version'

or I would also be happy with some semver check / regexp

[huitseeker]

For inspiration: privacy-scaling-explorations/halo2curves#59

[huitseeker]

TYSVM!!