Support decoding --reviewers as plain-text JSON

[fheinecke]

The shared-workflows bot currently only supports decoding the --reviewers flag as a base64-encoded JSON string. For example, rather than

{
  "reviewer-group-1": {
    "username": {
        ... // Repeat for 100+ lines

the tool requires the value as ewogICJyZXZpZXdlci1ncm91cC0xIjogewogICAgInVzZXJuYW1lIjogewogICAgICAgIC4uLiAvLyBSZXBlYXQgZm9yIDEwMCsgbGluZXMK.... For those of us who cannot base64-decode a string in their head, this is really difficult to read and debug.

Previously this was needed because GHA environment variables/secrets do not "play nice" with structured multiline data. However, now that we're starting to use env-loader for these values, we can support plain-text JSON without issue.

This change is fully backwards compatible with all current uses of the bot.

[rosstimothy]

Should this have test coverage?