Typed builders for stickers used by sticker-set methods.

Each builder fixes Telegram's format, requires 1-20 emoji strings, omits nil fields, and preserves explicit false values. Static stickers may use a file ID, HTTP URL, or Nadia.InputFile; animated and video stickers cannot use HTTP URLs.

:mask_position is meaningful only for mask sets. :keywords is meaningful only for regular and custom-emoji sets and accepts at most 20 strings whose combined length is at most 64 characters.