Initialize gateway wallet with specified ECDSA private key

[37ng]

Is your feature request related to a problem?

Currently it generates a new random wallet every time the gateway is initialized. We should be able to create a configurable wallet from a specified private key.

Describe the solution to the problem

Use a WALLET_PRIVATE_KEY environment variable and create a wallet from it during gateway initialization.

Describe the uses cases for the feature

We probably need to store this private key in AWS secret manager and pass its value into the container as a environment variable.
We could even use KMS to sign the txs instead of using in-memory wallet to boost the security, but that doesn't need to be prioritized for now.

Additional details

No response