Include current nonce in recommended-nonce route

[fmrsabino]

• Changes the endpoint route from GET /v1/chains/:chainId/safes/:safeAddress/recommended-nonce to GET /v1/chains/:chainId/safes/:safeAddress/nonces
• The returned successful response now contains the current Safe nonce under currentNonce.
• The recommended nonce is now returned in recommendedNonce.
• This route is now behind a feature flag. To enable this route, FF_NONCES_ROUTE should be set to true.

GET /v1/chains/:chainId/safes/:safeAddress/nonces

{
  "currentNonce": <number>,
  "recommendedNonce": <number>
}

[coveralls]

Pull Request Test Coverage Report for Build 6719080103

• 22 of 22 (100.0%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.01%) to 91.289%

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Totals
Change from base Build 6719026605:	-0.01%
Covered Lines:	4955
Relevant Lines:	5298

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💛 - Coveralls

[iamacook]

LGTM 🚀