Add method to set K8S custom environments akin to custom DSNs

[peterfarrell]

Currently, an error is reported to the correct DNS in Sentry, however it uses the environment set in SENTRY_ENVIRONMENT. This is very inflexible and limits you to a single environment.

We'd like the ability override the default environment and enable custom environment overrides akin to the SENTRY_K8S_CUSTOM_DSNS setting by adding a newSENTRY_K8S_CUSTOM_ENVIRONMENTS variable.

New Configuration Variable

• SENTRY_K8S_CUSTOM_ENVIRONMENTS - if set to 1, enables custom environments to be specified in the annotations with key k8s.sentry.io/environment which would take precedence over SENTRY_ENVIRONMENT. Disabled by default.

Example resource using SENTRY_K8S_CUSTOM_DSNS and SENTRY_K8S_CUSTOM_ENVIRONMENTS via annotations:

apiVersion: v1
kind: Pod
metadata:
  creationTimestamp: null
  labels:
    run: pod-crashloop
    type: test-pod
  name: pod-crashloop
  annotations:
    k8s.sentry.io/dsn: "https://xxxx@xxxx.ingest.us.sentry.io/xxxx"
    k8s.sentry.io/environment: "staging"
spec:
  containers:
    - image: alpine:3
      name: pod-crashloop
      command:
        - sh
        - "-c"
        - |
          sleep 1
          invalid_command
  dnsPolicy: ClusterFirst
  restartPolicy: Always