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.