[JS][Gemini] Allow safetySettings to be driven by a single string.

[mbleigh]

The granular safety settings of Gemini are pretty obnoxious, would be nice if you could set them to a blanket value that would be expanded to each category:

gemini15Flash.withConfig({
  safetySettings: "BLOCK_ONLY_HIGH",
})

// equivalent to
gemini15Flash.withConfig({
  safetySettings: [
    {
      category: 'HARM_CATEGORY_HATE_SPEECH',
      threshold: 'BLOCK_ONLY_HIGH',
    },
    {
      category: 'HARM_CATEGORY_DANGEROUS_CONTENT',
      threshold: 'BLOCK_ONLY_HIGH',
    },
    {
      category: 'HARM_CATEGORY_HARASSMENT',
      threshold: 'BLOCK_ONLY_HIGH',
    },
    {
      category: 'HARM_CATEGORY_SEXUALLY_EXPLICIT',
      threshold: 'BLOCK_ONLY_HIGH',
    },
  ]
});