Chart: Improve `git-sync` support

[kaxil]

Following things needs to be improved in the Helm Chart before we can release it to better support git-sync:

• There should be a way to pass additional environment variables (listed here) to git-sync containers instead of hard-coded list at

  airflow/chart/templates/_helpers.yaml

  Lines 136 to 192 in 303c89f

  	{{/* Git sync container */}}
  	{{- define "git_sync_container"}}
  	- name: {{ .Values.dags.gitSync.containerName }}
  	image: {{ template "git_sync_image" . }}
  	imagePullPolicy: {{ .Values.images.gitSync.pullPolicy }}
  	securityContext:
  	runAsUser: {{ .Values.dags.gitSync.uid }}
  	env:
  	{{- if .Values.dags.gitSync.sshKeySecret }}
  	- name: GIT_SSH_KEY_FILE
  	value: "/etc/git-secret/ssh"
  	- name: GIT_SYNC_SSH
  	value: "true"
  	{{- if .Values.dags.gitSync.knownHosts }}
  	- name: GIT_KNOWN_HOSTS
  	value: "true"
  	- name: GIT_SSH_KNOWN_HOSTS_FILE
  	value: "/etc/git-secret/known_hosts"
  	{{- else }}
  	- name: GIT_KNOWN_HOSTS
  	value: "false"
  	{{- end }}
  	{{ else if .Values.dags.gitSync.credentialsSecret }}
  	- name: GIT_SYNC_USERNAME
  	valueFrom:
  	secretKeyRef:
  	name: {{ .Values.dags.gitSync.credentialsSecret | quote }}
  	key: GIT_SYNC_USERNAME
  	- name: GIT_SYNC_PASSWORD
  	valueFrom:
  	secretKeyRef:
  	name: {{ .Values.dags.gitSync.credentialsSecret | quote }}
  	key: GIT_SYNC_PASSWORD
  	{{- end }}
  	- name: GIT_SYNC_REV
  	value: {{ .Values.dags.gitSync.rev | quote }}
  	- name: GIT_SYNC_BRANCH
  	value: {{ .Values.dags.gitSync.branch | quote }}
  	- name: GIT_SYNC_REPO
  	value: {{ .Values.dags.gitSync.repo | quote }}
  	- name: GIT_SYNC_DEPTH
  	value: {{ .Values.dags.gitSync.depth | quote }}
  	- name: GIT_SYNC_ROOT
  	value: {{ .Values.dags.gitSync.root | quote }}
  	- name: GIT_SYNC_DEST
  	value: {{ .Values.dags.gitSync.dest | quote }}
  	- name: GIT_SYNC_ADD_USER
  	value: "true"
  	- name: GIT_SYNC_WAIT
  	value: {{ .Values.dags.gitSync.wait | quote }}
  	- name: GIT_SYNC_MAX_SYNC_FAILURES
  	value: {{ .Values.dags.gitSync.maxFailures | quote }}
  	{{- if .is_init }}
  	- name: GIT_SYNC_ONE_TIME
  	value: "true"
  	{{- end }}
  	volumeMounts: