Make Bitbucket's sensitive configuration file-based

[skabashnyuk]

Is your task related to a problem? Please describe.

At this moment CHE_OAUTH1_BITBUCKET_CONSUMERKEY and CHE_OAUTH1_BITBUCKET_PRIVATEKEY configured as environment variables
or configuration properties. We would like to move these values to the file to make them more secured.

Describe the solution you'd like

Sensitive configuration should be configured with Secrets and mounted eclipse-che/che-operator#595 with che-operator as a file to che-server container.
Example:

apiVersion: v1
kind: Secret
metadata:
  name: che-server-settings-secret
  labels:
    app.kubernetes.io/part-of: che.eclipse.org
    app.kubernetes.io/component: che-secret
  annotations:
    che.eclipse.org/mount-path: /home/user/che-server/settings/
data:
  bitbucket_consumer_key: <base64 encoded data content here>  
  bitbucket_private_key: <base64 encoded data content here>  

  @Inject
  public BitbucketServerOAuthAuthenticator(
      @Named("/home/user/che-server/settings/bitbucket_consumer_key") File consumerKey,
      @Named("/home/user/che-server/settings/bitbucket_private_key") File privateKey,
      @Named("che.oauth1.bitbucket.endpoint") String bitbucketEndpoint,
      @Named("che.api") String apiEndpoint) {
    super

TBD

1. Concrete che.eclipse.org/mount-path
2. @Named("file:/home/user/che-server/settings/bitbucket_private_key") String privateKey - injection of file values.
3. che.oauth1.bitbucket.endpoint - configure with file too.

Describe alternatives you've considered

n/a

Additional context

#18385

[skabashnyuk]

Done as part of #18726