[FEAT] Implement a testnet environment for MTE

[Dom-Mac]

Implement a testnet environment for MTE

Description

Merge to earn right now is available only on Mainnet.
That's because there are risks involved in linking a repo to a testnet contract, as contributors may be misled to believe that it's on a mainnet.
At the same time a testnet could be useful to try MTE features.

Spec

Supporting a testnet adds some complexities that are not handled right now.
Things to keep in mind:

• The chain must appear in the bots comments to provide transparency to contributors
• MTE GitHub app installations are unique for each repo. Each repo is associated to a slicer and multisig.
• Each repo may be also linked to a specific chain, to allow support for testnet while being future-proof