[fluent-bit] Load configuration from either ConfigMap or Secret

Currently it is not possible to load the configuration of input and/or output to come from a `Secret`. There are cases where passwords or connection string are used and they will then be saved as a `ConfigMap` which is not really ideal. Additionally if it becomes possible to provide `existingSecret`, it would be possible to get the secret from an external source, e.g. a Vault via VSO.

Currently I've edited my personal chart to just use a `Secret`, but I can take over this issue if triaged and make it personalized.

I would say similarly to `kind`, `configKind` can be added to be set as either `ConfigMap` or `Sercret`, defaulting to `ConfigMap` to prevent huge breaking changes. Then either a `ConfigMap` or `Secret` is created, with the possibility to provide a `existingSecret` as well.

Inspect the fluent-bit chart's values and resource templates, starting with how the current configuration kind is selected and how ConfigMaps are created. The work is done when configuration can use either a ConfigMap or Secret, an existing Secret can be referenced, and the default remains ConfigMap without breaking current users.

Written by the indexing model from the issue text.