Add optional safeTxGas feature

[iamacook]

To match the new feature in the chains endpoint the FEATURES enum was extended to add SAFE_TX_GAS_OPTIONAL.

[katspaugh]

Thanks 👍

[github-actions]

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Type	Occurrences	Fixable
Errors	0	0
Warnings	0	0
Ignored	0	N/A

• Result: ✅ success
• Annotations: 0 total

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^{Report generated by eslint-plus-action}

[katspaugh]

Wait, this feature doesn't belong here. It's not per chain, it's per Safe version.

[iamacook]

Why is this then alongside the same details that are to be returned from the endpoint? No versioning was discussed alongside features. The intention is to remain agnostic as possible.

[katspaugh]

Reopened because we still need to define this feature in the enum.