Replaces the string-based `tones` field with a type-safe
`buttons: list[KeypadEntry]` on `OutputDTMFFrame` and
`OutputDTMFUrgentFrame`, matching the existing singular `button`
field on `InputDTMFFrame`. A `from_string` classmethod builds the
list from a dial string like `"123#"` (invalid characters raise
ValueError from the `KeypadEntry` constructor).
The base output audio fallback now iterates `frame.buttons`
directly, LiveKit sends `frame.buttons[0].value`, and the Daily
transport joins the button values into the single string Daily's
`send_dtmf` expects.
Introduces a new `tones` field on `OutputDTMFFrame` and
`OutputDTMFUrgentFrame` for sending multi-digit DTMF sequences and
deprecates the existing single-key `button` field. When only `button`
is set, it is used as a single-character `tones` string for backward
compatibility.
`DTMFFrame` is kept as an empty marker class so both input and output
DTMF frames can still be identified via isinstance. `InputDTMFFrame`
keeps its required `button` field (single keypress semantics).
The Daily-specific `DailyOutputDTMFFrame` and
`DailyOutputDTMFUrgentFrame` frames no longer need to override
`button` and simply add `session_id` and `digit_duration_ms`, which
are forwarded to Daily's `send_dtmf` as `sessionId` and
`digitDurationMs`.
The base output audio fallback now iterates `tones` and generates a
tone per character; LiveKit's native DTMF path sends `tones[0]` since
its API is single-tone.
Introduces Daily-specific DTMF output frames that carry explicit
`tones`, `session_id` and `digit_duration_ms` fields, forwarded to
Daily's `send_dtmf` as `tones`, `sessionId` and `digitDurationMs`.
The inherited `button` and `transport_destination` fields are
ignored for these frames in the Daily transport.
