Add possibility to provide config.toml in helm chart

[hypnoglow]

Is your feature request related to a problem? Please describe.

Currently there is no way to provide custom config.toml file to Athens when using helm chart. All configuration is done via environment variables. However, in some cases config file is preferable. And in some edge cases environment variables won't work (for example, see gomods/athens#1404 )

Describe the solution you'd like

We should provide configuration file with customization possibility in helm chart. This can be done via exposing a value in values-file, e.g.:

config.toml: |-
    GoEnv = "development"
    GoBinaryEnvVars = ["GOPROXY=direct"]

Default value should be equal to the default config which is included in the docker image.

Another option would be allowing extraVolumes and extraVolumeMounts, as done in many popular helm charts (e.g. vault. This way user is responsible for providing their config files, placing them in correct locations and mounting properly.