[V107-specific] Add withdrawal transaction Asset to its pool, if existing

[V107-specific]

Intended to close #1415 (closed)
'External chain withdraw-initiator transactions' coins are adding to vault solvency without being added to their pool'.

As always, feedback is welcome.

According to #1415 (closed)'s

plausibly Midgard might need an update to record BalanceAsset changes from withdraw events after the change?

, though smoke tests pass I wanted to try making a Midgard branch that (version-dependently?) records the pool depth change from the "coin" field of the "withdraw" event.

Based on this thread, my current impression is that this could need a retroactive Midgard update after THORNode implementation, specifically referencing the implementation block height.

Relevant:
https://gitlab.com/thorchain/midgard/-/blob/2.13.0/internal/fetch/record/correctmainnet.go#L34
https://gitlab.com/thorchain/midgard/-/blob/2.13.0/internal/fetch/record/record.go#L470-489

When adding that I might want to make a comment about that, rather than in block 1970000,
the previous code change mentioned was made in block 2104917 (2021-09-15) upon the switch from v0.64.0 to v0.67.0,
specifically the implementation of !1834 (merged) (resolving #1052 (closed)).