This commit extends the chisel release with keyring definitions.
Keyrings are defined in ASCII armored format in the top-level
public-keys property by name. Keyrings are referenced by name in the
public-keys list property in archive definitions. An example of the
extended chisel release file is at the bottom.

This commit uses the newly added github.com/ProtonMail/go-crypto/openpgp
package dependency[1]. This package is a maintained fork of the
deprecated golang.org/x/crypto/openpgp package[2][3].

[1] https://github.com/ProtonMail/go-crypto
[2] https://pkg.go.dev/golang.org/x/crypto/openpgp
[3] https://golang.org/issue/44226

Example chisel.yaml:

    format: chisel-v1
    archives:
      ubuntu:
        version: 22.04
        components: [main, universe]
        suites: [jammy, jammy-updates, jammy-security]
        public-keys: [ubuntu]
      ubuntu-fips:
        version: 22.04
        pro: fips
        components: [main]
        suites: [jammy]
        public-keys: [ubuntu-fips]
    public-keys:
      ubuntu: |
        -----BEGIN PGP PUBLIC KEY BLOCK-----

        mQINBFzZxGABEADSWmX0+K//0cosKPyr5m1ewmwWKjRo/KBPTyR8icHhbBWfFd8T
        DtYggvQHPU0YnKRcWits0et8JqSgZttNa28s7SaSUTBzfgzFJZgULAi/4i8u8TUj
        +KH2zSoUX55NKC9aozba1cR66jM6O/BHXK5YoZzTpmiY1AHlIWAJ9s6cCClhnYMR
        ...
        E+SWDGxtgwixyPziL56UavL/eeYJWeS/WqvGzZzsAtgSujFVLKWyUaRi0NvYW3h/
        I50Tzj0Pkm8GtgvP2UqAWvy+iRpeUQ2ji0Nc
        =j6+P
        -----END PGP PUBLIC KEY BLOCK-----
      ubuntu-fips: |
        -----BEGIN PGP PUBLIC KEY BLOCK-----

        mQINBE+tgXgBEADfiL1KNFHT4H4Dw0OR9LemR8ebsFl+b9E44IpGhgWYDufj0gaM
        /UJ1Ti3bHfRT39VVZ6cv1P4mQy0bnAKFbYz/wo+GhzjBWtn6dThYv7n+KL8bptSC
        Xgg1a6en8dCCIA/pwtS2Ut/g4Eu6Z467dvYNlMgCqvg+prKIrXf5ibio48j3AFvd
        ...
        mguPI1KLfnVnXnsT5JYMbG2DCLHI/OIvnpRq8v955glZ5L9aq8bNnOwC2BK6MVUs
        pbJRpGLQ29hbeH8jnRPOPQ+Sbwa2C8/ZSoBa/L6JGl5RDaOLQ1w=
        =6Bkw
        -----END PGP PUBLIC KEY BLOCK-----

This commit builds on the previous commit that introduces support for
OpenPGP keyrings to verify signatures of archive release files.
Previously, we used the Release file of each configured suite. With this
commit, we fetch the InRelease file instead. This file is in clearsign
format[1] and contains the Release file and its signature[2].

[1] https://www.gnupg.org/gph/en/manual/x135.html
[2] https://www.chiark.greenend.org.uk/~cjwatson/blog/no-more-hash-sum-mismatch-errors.html