Support reading configuration from config files

[romanwozniak]

Motivation:

As of now, all the configuration values are passed to gojek/mlp via environment variables. With the growing complexity of the application, the complexity of its configuration grows proportionally, so this way of passing config data to the application is no longer convenient. This is because it requires some sort of encoding to pass complex struct/object data (i.e. serializing data as JSON-string. Example: charts/mlp/values.yaml#L47) to the application.

This PR does the necessary refactoring to allow passing runtime configuration to the application via YAML config files, while still making it possible to override sensitive config keys via env variables:

mlp --config config-dev.yaml --config config-overrides.yaml

Changes:

Internally, knadh/koanf is used to parse and deserialize config data. Despite some similarity with spf13/viper, knadh/koanf doesn't lowercase config keys and generally feels like a better-maintained alternative to viper.

Breaking Changes:

Streams/Teams configuration was refactored to represent the Stream > Team hierarchy. Previously Stream and Team were two independent slices of strings, so MLP project can be created with any stream/team combination, which breaks the one-to-many relationship between stream and team.
Now, the Team configuration is completely removed from the top-level configuration and instead, the type of Stream configuration is changed to map[string][]string to hold the mapping between streams and the teams belonging to that stream.
Before:

Streams:
  - stream-a
  - stream-b

Teams:
  - team-a
  - team-b

After:

Streams:
  "stream-a":
    - team-a
  "stream-b":
    - team-b

Applications' UI was updated to support this refactoring too.

[krithika369]

Left some questions for clarification. LGTM. Thanks!