Passive Mode Agent

Cerated out of #1757

Recent events have shown that the agent needs to be able to run in a safe mode where it maintains the current position but does not engage in new positions.
The main reason for it is, that the agent is required to have a certain amount of ETH to function properly. But currently the agent is designed to only spend ETH, eventually depleting its ETH funds.

With the minimum source balance, an LP could already mitigate entering new positions but I consider this more of a hack.

## Passive Mode
I see the spec of the passive mode as follows:

* Don't fill any new requests
* Continue to run the agent
* Claim/Challenge/withdraw should be executed like in the normal mode
* Agent should not enter new challenges
* Agent should log if their are no outstanding transactions anymore and exit
* If Agent starts in passive mode and there are no outstanding transactions sam as the above
* Flag in the config to enable passive mode
* Should ignore the whitelist check

Also, add documentation for some scenarios e.g. agent just wants to shut down the agent for a certain time or wants to exit the system etc.

The issue does not name files, tests, or entry points. Start by tracing the agent configuration and transaction handling, then define how passive mode affects filling, challenges, claims, withdrawals, whitelist checks, shutdown logging, and documentation for temporary shutdown or exit scenarios.

Written by the indexing model from the issue text.