Feature: Ethers signer

[mmv08]

This PR:

• Adds SafeAppsSdkSigner to safe-apps-ethers-provider package which allows calling smart contract functions just as you would in a dapp: exampleContract.func()

See in action: rmeissner/sapp-phenix#2

[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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[github-actions]

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