Add ability to configure liveness and readiness probes

[huseyinbabal]

Description

Changes proposed in this pull request:

• Added configurable liveness+readiness probe

Testing

helm upgrade --install botkube \
  --version v1.0.0 \
  --namespace botkube \
  --create-namespace \
  --wait \
  --set config.provider.endpoint=https://api-dev.botkube.io/graphql \
  --set config.provider.identifier=... \
  --set config.provider.apiKey=... \
  ./helm/botkube

And check liveness + readiness probe, they should have default values as described in values.yaml. You can also customize it as shown below

deployment:
  livenessProbe:
    periodSeconds: 120
  readinessProbe:
    periodSeconds: 120

helm upgrade --install botkube \
  --version v1.0.0 \
  --namespace botkube \
  --create-namespace \
  --wait \
  --set config.provider.endpoint=https://api-dev.botkube.io/graphql \
  --set config.provider.identifier=... \
  --set config.provider.apiKey=... \
  ./helm/botkube -f /tmp/bk-cfg.yaml

Related issue(s)

Fixes #1055

[pkosiec]

LGTM - just please apply suggestion to improve the comment 🙂
And let's tweak the default values a bit. Thanks!