feat: add tx hash to retirements table

[wgwz]

Closes: regen-network/rnd-dev-team#1753

This adds the tx hash to each row in the retirements table.
This is done in such a way where it can be re-used in the future.
I.e. we can include the tx hash in the proposals PR as well.

[wgwz]

One thing to note is that either we need an additional/modified migration or as part of deploying this we could truncate the retirements table.
The problem is that the new tx hash column needs to be populated with correct data.
I'd suggest truncating the retirements table before applying the migration in this PR and deploying.
It's a simple approach with little downsides, it would take a couple of minutes for all retirements to be reindexed.

[blushi]

Truncating the retirements table sounds fine to me