feat!: improve signing DX

[danielbate]

Closes #1745

• [x] Implementing signing transaction via an Account through a wallet connector
• [x] Add multiple external signers to a request through addAccountWitnesses functions on BaseTransactionRequest
• [x] Rename createWitness to addEmptyWitness and implement addWitness fully
• [x] Add documentation and example for a script and predicate multi sig with external signers (we only ever used signMessage, I have added signTransaction examples)
• [x] Improvements and fixes to current docs around signing

Please note, the approach discussed in #1576 has not been implemented here however there is merit to it, as shown in the predicate test example.

[Dhaiwat10]

@danielbate there are some forc formatting errors that you might want to look at

[danielbate]

cc @calldelegation

[github-actions[bot]]

✨ A PR has been created under the rc-1815 tag on fuels-wallet repo. https://github.com/FuelLabs/fuels-wallet/pull/1174

[github-actions[bot]]

Coverage Report:

Lines	Branches	Functions	Statements
81.35%(-0.11%)	71.84%(-0.14%)	78.98%(-0.17%)	81.46%(-0.1%)

Changed Files:

Ok	File (✨=New File)	Lines	Branches	Functions	Statements
🔴	packages/account/src/account.ts	76.51% (-1.78%)	53.96% (-1.77%)	81.81% (-3.9%)	76.51% (-1.78%)
🔴	packages/account/src/connectors/fuel-connector.ts	25.92% (-1%)	100% (+0%)	4.76% (-0.24%)	25.92% (-1%)
🔴	packages/account/src/providers/provider.ts	78.14% (-0.21%)	64.12% (-0.44%)	85.07% (+0%)	78.85% (-0.21%)
🔴	packages/account/src/providers/transaction-request/transaction-request.ts	85.61% (+0.61%)	69.44% (+0.87%)	86.27% (+0.86%)	85.43% (+0.61%)