[Early Draft][Splicing] Interactive tx negotiation, with a single ChannelContext.

[optout21]

This is a continuation of #3444, without duplication of ChannelContext, only `FundingScope' is duplicated during splice negotiation.

Includes:

• [Splicing] Partial, handle splice_init & splice_ack messages #3407
• Dual funding extension, needed also for splicing: begin_interactive_funding_tx_construction #3443
• Extend begin_interactive_funding_tx_construction() with splicing-specific extra input (on top of #3443) #3516

[optout21]

Status: it is mostly implemented, in a rough form, but it's not fully functional, due to some fields needing to be moved to FundingScope.

• SplicingChannel fields: SplicingChannel should not include full channel structs, as in [Draft][Splicing] Partial, perform interactive transaction negotiation for new funding #3444, but the constituent members of FundedChannel (the pre-splice funded channel), and PendingV2Channel (the new V2 being negotiated), but without duplicating channel context. Essentially, sg. like this:

	/// Fields from FundedChannel
	pub pre_funding: FundingScope,
	pub context: ChannelContext<SP>,
	pre_holder_commitment_point: HolderCommitmentPoint,
	pending_splice_pre: PendingSplicePre,
	pending_splice_post: PendingSplicePost,

	// Fields from PendingV2Channel follow, except ChannelContext, which is reused from above
	pub post_funding: FundingScope,
	pub unfunded_context: UnfundedChannelContext,
	/// Used when negotiating the splice transaction
	pub dual_funding_context: DualFundingChannelContext,
	/// The current interactive transaction construction session under negotiation.
	pub interactive_tx_constructor: Option<InteractiveTxConstructor>,

• Avoid duplications 1. Since channel fields are duplicated, some of its methods need to be duplicated as well. First trick to reduce is to include the full FundedChannel (instead of its fields):

	pub pre_funded: FundedChannel<SP>,

	// Fields from PendingV2Channel follow, except ChannelContext, which is reused from above
	pub post_funding: FundingScope,
	pub unfunded_context: UnfundedChannelContext,
	/// Used when negotiating the splice transaction
	pub dual_funding_context: DualFundingChannelContext,
	/// The current interactive transaction construction session under negotiation.
	pub interactive_tx_constructor: Option<InteractiveTxConstructor>,

• Avoid duplications 2. Method duplications can be avoided by making FundedChannel and PendingV2Channel traits, with some common implementations (accessing fields through accessiors), and making FundedChannel, PendingV2Channel, and SplicingChannel implementing relevant accessors. This is not yet done.

• Functional issue, failure during commitment exchange.
  The process fails somewhere around commitment exchange, with a force close. The reason is that some fields which were duplicated and changed in [Draft][Splicing] Partial, perform interactive transaction negotiation for new funding #3444, are not duplicated currently.

These fields can be seen in new_for_splice() and new_spliced(). They are:

• funding_tx_confirmed_in
• funding_tx_confirmation_height
• channel_transaction_parameters.funding_outpoint
• channel_state
• next_funding_txid
• latest_monitor_update_id
• channel_pending_event_emitted
• channel_ready_event_emitted
• blocked_monitor_updates

Note that some of the most important fields have been already moved to FundingScope:

• channel_value_satoshis
• value_to_self_msat
• holder_max_commitment_tx_output
• counterparty_max_commitment_tx_output
• funding_transaction -- Refactor ChannelTransactionParameters into FundingScope #3604
• holder_max_commitment_tx_output -- Refactor ChannelContext value fields into FundingScope #3592