Supports passing exchange pair's rate as string

[unnawut]

Issue/Task Number: #996
Closes: #996

Overview

This PR add supports for exchange pair's creation and update by passing the rate as a string.

Changes

• Attempts to parse a string rate into float in EWallet.ExchangePairGate

Implementation Details

This is a small update to the ExchangePairGate that tries to parse a string rate before doing the insert/update.

A caveat is that this uses Float.parse/1 which means that there could be precision loss in the floating point returned. But since the DB is already storing the rate as a floating point, this does not introduce more precision loss than it already is. The precision loss is #998.

Usage

curl -X POST localhost:4000/api/admin/exchange_pair.create \
-H "Accept: application/vnd.omisego.v1+json" \
-H "Authorization: OMGAdmin <truncated>" \
-H "Content-Type: application/json" \
-d '{
  "from_token_id":"tok_ETH_01d7brbvs76ceqyp683enns91a",
  "to_token_id":"tok_KNC_01d7brbvnftq9efg039emzbch9",
  "rate":"1000",
  "sync_opposite":true
}' \
-v -w "\n" | jq

Impact

• No changes to DB schemas.
• API specs is updated with a non-breaking change (allow exchange pair request bodies to have rate that is either string or number).