Bitcoin difficulty maintainer for keep-test

Here we update the configuration of bitcoin maintainer. We use the
keep-maintainer template and run the `bitcoinDifficulty` module of the
maintainer connected to mainnet Bitcoin network.

This is expected to be a temporary solution that will be updated to
overcome the limits of LightRelay and Bitcoin testnet integration
mentioned in the README

Author: nkuba

infrastructure/kube/keep-test/btc-difficulty/kustomization.yaml

@@ -0,0 +1,14 @@
+# Keep Maintainer Staging Mainnet
+
+Bitcoin Relay used by the tTBTC system on testnet environment is an optimized version
+of the Light Relay implemented by [GoerliLightRelay](https://github.com/keep-network/tbtc-v2/blob/main/solidity/contracts/test/GoerliLightRelay.sol)
+contract. The reason the Light Relay had to be modified for testnet is that on
+Bitcoin testnet difficulty often drops to `1`, which makes the blocks validation
+on such change impossible to the regular Light Relay contract.
+
+The `GoerliLightRelay` version doesn't require a maintainer bot to submit block
+headers on difficulty change. It accepts ad-hoc difficulty alignment according to
+the tests needs.
+
+The setup defined in this directory is meant for testing the bitcoin difficulty module 
+of the maintainer bot with the Bitcoin mainnet blockchain state.

infrastructure/kube/keep-test/btc-difficulty/maintainer-statefulset.yaml

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+resources:
+  - ../../templates/keep-maintainer
+
+namespace: default
+
+# Regular testnet maintainer doesn't run bitcoinDifficulty module due to specifics
+# of LightRelay running on testnet (GoerliLightRelay). This setup is running
+# the `bitcoinDifficulty` module of maintainer connected to Bitcoin mainnet.
+nameSuffix: -staging
+
+commonLabels:
+  app: keep-maintainer-staging
+  type: bitcoinDifficulty
+  network: goerli
+
+images:
+  - name: keep-maintainer
+    newName: gcr.io/keep-test-f3e0/keep-client
+    newTag: latest
+
+configMapGenerator:
+  - name: keep-maintainer-config
+    behavior: merge
+    literals:
+      - network=testnet
+      - electrum-api-url=wss://electrumx-server.tbtc.network:8443
+    files:
+      - .secret/keep-maintainer-keyfile
+
+secretGenerator:
+  - name: keep-maintainer-eth-account-password
+    files:
+      - .secret/keep-maintainer-password
+
+patches:
+  - target:
+      kind: StatefulSet
+      name: keep-maintainer
+    patch: |-
+      - op: add
+        path: /spec/template/spec/containers/0/args/-
+        value: --bitcoinDifficulty
+      - op: replace
+        path: /spec/template/spec/containers/0/env/0/valueFrom/secretKeyRef/name
+        value: eth-network-goerli
+
+generatorOptions:
+  disableNameSuffixHash: true
+  annotations:
+    note: generated