Add a context to interactive tx session

[pm47]

There are three closely related, but separate parameters:

• InteractiveTxParams: low-level bitcoin parameters (amounts, outputs, locktime, confirmations)
• Purpose: whether the interactive-tx is an initial funding, a splice, a rbf
• SessionContext: higher level meta information about the interactive-tx (why are we opening or splicing this channel?)

This PR adds a SessionContext and carries it all the way to the completion (success or failure) of the interactive-tx process. It is useful for better debugging, diagnostics, metrics.

[t-bast]

I don't think this will be necessary after #2848, which adds a liquidityPurchase_opt field to InteractiveTxBuilder. If liquidity ads is used, this field will be filled. Otherwise, the channel funding / splice will have been manually initiated by the node operator.

Every on-the-fly funding made to wallet nodes will simply be a liquidity purchase, and the details of what triggered it will be contained in the paymentDetails field.

[t-bast]

Closing this in favor of #2848