secret: add different formats

[Codelax]

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This present an idea to improve secret api by adding new fields.

The first one would be adding a base64 field to allow uploading files or anything that is not valid utf-8

resource scaleway_secret_version version {
  data = "string"
  data_base64 = "base64encoded_string"
}

For the datasource, we could add a type field that default to base64.

data scaleway_secret_version version_base64 {
  # type = "base64"
  # data = "base64"
}

data scaleway_secret_version version_base64 {
  type = "string"
  # data = "valid string"
}