Add ENCODE_KEY helm values inputs

We recently found out the hard way about the `encKey` used to encrypt things in the database. It would be helpful for users deploying on kubernetes if the default values file contained references to this feature so that it is more obvious that it can be set (rather than generated by a container that may not persist).

My suggestion is to add inputs in the values file like:
```yaml
# This is the string used to encrypt sensitive things like PATs in the database.
# Alternatively, you may supply this value to the `lake` container directly as environment variable `ENCODE_KEY`
# If unset, a key will be created dynamically, in which case you should retrieve it and store it somewhere secure and persistent
encodeKey:
secretName: "" # the name of the Secret containing this encryption key
secretKey: "" # the name of the key within that Secret which contains the encryption key as its value
```