Add a new reserved parameter to allow minConfirmations to be specified

[bbenligiray]

Currently, an Airnode can be specified to use a global minConfirmations value for each chain. Accordingly, the AQN Airnode has been configured to use reasonable values for the expected use-case.

Both with QRNG and other potential Airnode services, users will want to customize this value. With normal oracle nodes/protocols (that fulfill all requests with the same wallet) this is not possible, but with Airnode it is. We can have a _minConfirmations reserved parameter that expects a number, and applies that only to the specific sponsor wallet. There are some caveats with this:

• Consider the example where the Airnode receives a request with a _minConfirmations of 10, and a second request with a _minConfirmations of 0 9 blocks later. This will cause a race condition and potentially cause different fulfillments to be submitted with the same nonce. To avoid this, we always need to respond to requests in the same nonce order as we receive them. In other words, Airnode should find the maximum _minConfirmations value in the entire queue and use that for the sponsor wallet.
• _minConfirmations shouldn't be larger than blockHistoryLimit.
• As all reserved parameters, the node should ignore _minConfirmations if it isn't defined in the specific endpoint (to prevent potential abuse).

[bbenligiray]

Another task need to have been created when Dave relayed on #technical-questions that a user wants a similar reserved parameter for gas price

[dcroote]

I can take this task. @bbenligiray would you like me to also add a gas price reserved parameter while at it per your above comment? If so I'll create that task and assign myself.

[bbenligiray]

@dcroote Sure, I assume you already saw #1492