data-plane requires public and private key settings also if not really needed

[ndr-brt]

Bug Report

Describe the Bug

Currently to start a data-plane is always mandatory to provide private and public key configuration through settings:
edc.transfer.proxy.token.signer.privatekey.alias
edc.transfer.proxy.token.verifier.publickey.alias

but they are only needed if the data-plane provides PULL transfer mechanism

Expected Behavior

to not require them in case of no PULL transfer mechanism is registered

Observed Behavior

exception at startup

Steps to Reproduce

build a barebone data-plane runtime and run it

Context Information

EDC 0.8.1

Detailed Description

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Possible Implementation

You already know the root cause of the erroneous state and how to fix it? Feel free to share your thoughts.