Support keystores in vm.sign

[matmilbury]

Component

Forge

Describe the feature you would like

Currently, signing with vm.sign is only possible with plain text private key.

An ability to use it with keystore would open a possibility of using safer keystores in scripts that need to sign messages.

For example, forge-safe could use vm.sign instead of ffi.

Additional context

Related:

• refactor: wallet management #7141
• feat(forge script): preview, batch, and execute Gnosis Safe transactions (ape-safe) #1232
• feat(cheatcodes): allow forge script to access wallet addresses with vm.getScriptWallets() #7213