feat: Revert "feat: Removed Gaia Liquid staking module"

[pbukva]

Reverts #426

[sentry]

Bug: Variable shadowing with := inside the IterateValidators callback causes errors from SetLiquidValidator to be silently ignored during the upgrade migration.
_{Severity: HIGH}

Suggested Fix

To ensure errors are handled, either panic on error within the callback by using panic(err), which matches patterns elsewhere in the codebase, or capture the error in a closure variable declared outside the callback. This captured error can then be checked after the iteration completes.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent.
Verify if this is a real issue. If it is, propose a fix; if not, explain why it's not
valid.

Location: app/upgrades.go#L132-L135

Potential issue: In the v0.53 upgrade handler, a new `err` variable is declared within
the `IterateValidators` callback using `:=`, shadowing the outer `err` variable.
Consequently, any error returned from `app.LiquidKeeper.SetLiquidValidator` is assigned
to this inner variable and then discarded, as the callback does not propagate errors.
This causes the error to be silently swallowed. If `SetLiquidValidator` fails for any
validator during the one-time migration, that validator will lack a `LiquidValidator`
record, leading to incomplete data for the liquid staking module and potential runtime
issues post-upgrade.

_{Did we get this right? 👍 / 👎 to inform future reviews.}