Move the signing keys info for each repo channel to repo json file

[agnostic-apollo]

Move the signing keys info for each repo channel to repo.json instead of hardcoding it in build scripts

This will allow each repo channel to define its own keys that may be used for verifying signatures of its files. The termux-packages fork will just need to add signing keys info in the repo.json file itself for any additional channels instead of having to patch build scripts. With the old hardcoded way, the unnecessary pacman key was also being added to local keystore in the current/upstream termux-packages repo, even though no repo channel uses it, so defining the keys in the repo.json will solve that issue as well.

The termux_repository__add_repo_signing_keys_to_keystore() function now handles the logic for adding the repository signing keys to the local keystore. Check its function docs for more info on the new json format and requirements for repo.json file.

---

Add repository signing keys to local keystore.
.
Each repo channel dict in `repo.json` file should have a
`signing_keys` key for an array of dicts, where each dict contains
information for each key that may be required by the parent repo
channel. At least one signing key info must exist for each repo
channel.
.
The `key_file` key should be set to the path of the key file. If it
is a relative path that does not start a slash `/`, then
`termux-packages` repo root directory will be prepended to it.
.
#
A dict is being used because other keys may be required
in future in addition to current `key_file` key, like `key_format`, etc.
.
```
"<repo_channel_path>": {
    "signing_keys": [
      {
        "key_file": "path/to/key1.gpg"
      },
      {
        "key_file": "path/to/key1.gpg"
      }
    ]
}
```
.
.
**Parameters:**
`repo_json_file` - The path to the `repo.json` file.
`repo_root_dir` - The path to the `termux-packages` repo root
                  directory.
.
**Returns:**
Returns `0` if successful, otherwise returns with a non-zero exit code.
.
.
termux_repository__add_repo_signing_keys_to_keystore `<repo_json_file>`
    `<repo_root_dir>`

---

1. @Maxython You will need to make changes to https://github.com/termux-pacman/termux-packages/blob/master/repo.json and add packages/termux-keyring/termux-pacman.gpg to each repository channel.
2. @licy183 You can remove the patch and make required changes to the https://github.com/termux-user-repository/tur/blob/master/repo.json file itself.

[Maxython]

One thing, I changed the method of getting information from repo.json in properties.sh file. Because I noticed that when trying to get keys of values, jq sorts them. Because of this it can indicate wrong data by index.