Netexec: injector to create its own signatures for inject expectations

[antoinemzs]

Use case

Netexec injector will create and send expectation signature per target as per OpenAEV-Platform/openaev#5447 (comment)

Netexec contracts need an additional ContractOutputElement to allow for connecting signatures structured output with an output processor on the backend side. Dependency on: OpenAEV-Platform/client-python#206

DoD

• All Netexec contracts have the new ContractOutputElement of type ExpectationSignature
• Netexec parses the inject contract and contents and creates structured output objects for each target and expectations (read list of expectations in contents). Follow example json in [signature redesign]: Implement Structured output processor behaviour in backend openaev#5447 (comment)
• Structured output objects may be chunked as needed to stay below a certain payload size for the callback endpoint
• Structured output objects may be sent in any order
• Netexec injector maintains a private list of relevant signature types to create for any target