Make SendOption exportable

[kukymbr]

Hi,

tbot seems to be useful and compact wrapper around the telegram API.

I'm proposing to make the sendOption type exportable, because there could be cases when some message factory is used before sending.

For example, on my project I have some message types (like MarkdownMessage, ErrorMessage, etc), the Sender interface and implementation for multiple IM's. And the TelegramSender needs to build message options dynamically (example from current implementation for a telebot library):

to, toOpts := recipientToTelebotRecipient(recipient)
opts := make([]any, 0, len(toOpts)+len(arg.opts))

opts = append(opts, arg.opts...)
opts = append(opts, toOpts...)

msg, err := bot.Send(to, arg.what, opts...)

But it's not possible if options are represented by an unexportable type.