[Vertex AI] Add URI-based file data support

[andrewheard]

Added support for URI-based file data (in addition to existing support for data as inline bytes). Files may be uploaded using the Firebase Storage SDK.

Example Usage:

// Obtain a reference to the file in Firebase Storage.
let reference = Storage.storage().reference(withPath: "/image.jpg")
let imagePart = ModelContent.Part.fileData(
  mimetype: "image/jpeg",
  // Construct a gs:// URI from the bucket name and full path
  uri: "gs://\(reference.bucket)/\(reference.fullPath)"
)

#no-changelog

[paulb777]

Tests?

[andrewheard]

Tests?

@paulb777 The real test is whether the backend accepts it but I added an encoding test to make it easier to visually inspect the JSON representation (since we have custom Encodable conformance it'd be easy to miss something). Also added a simple API test.