chore: create base_token_ratios_ratio_timestamp_idx index (#3411)

## What ❔

This PR adds a migration that creates an index on `ratio_timestamp`
field of `base_token_ratios` table.

## Why ❔

Queries to this table are currently much heavier than they should be,
that has to be optimized.

## Checklist

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- [ ] PR title corresponds to the body of PR (we generate changelog
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- [ ] Tests for the changes have been added / updated.
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lint`.

Author: dimazhornyk

core/lib/dal/migrations/20241226161705_add_ratio_timestamp_index.down.sql

@@ -0,0 +1 @@
+DROP INDEX IF EXISTS base_token_ratios_ratio_timestamp_idx;

core/lib/dal/migrations/20241226161705_add_ratio_timestamp_index.up.sql

@@ -0,0 +1 @@
+CREATE INDEX IF NOT EXISTS base_token_ratios_ratio_timestamp_idx ON base_token_ratios (ratio_timestamp);