Add support for storing Google Cloud key as ENV var directly

[weilandia]

This is a little verbose, but was in a hurry and wanted the fewest diffs.

Being able to use the key directly is much simpler for multi env production apps so we don't have to deal with files, especially when we only use Google Cloud for Vertex.

[gbaptista]

@weilandia, thanks for opening the pull request!

Yeah, I got it. So, I'm thinking about the name of the key. json_key works but can be a bit confusing. I was thinking about keeping the semantics of file and using file_contents so it can match the already existing file_path:

client = Gemini.new(
  credentials: {
    service: 'vertex-ai-api',
    file_path: 'google-credentials.json',
    region: 'us-east4'
  },
  options: { model: 'gemini-pro', server_sent_events: true }
)

So, it would be:

client = Gemini.new(
  credentials: {
    service: 'vertex-ai-api',
    file_contents: File.read('google-credentials.json'),
    region: 'us-east4'
  },
  options: { model: 'gemini-pro', server_sent_events: true }
)

client = Gemini.new(
  credentials: {
    service: 'vertex-ai-api',
    file_contents: ENV['GOOGLE_CREDENTIALS_FILE_CONTENTS'],
    region: 'us-east4'
  },
  options: { model: 'gemini-pro', server_sent_events: true }
)

How do you feel about it?

[weilandia]

Sounds good, I updated the PR.

[gbaptista]

@weilandia Awesome, thanks for your contribution!