Allow to disable env variable expansion by setting DEX_EXPAND_ENV = false

[heidemn-faro]

Is your feature request related to a problem? Please describe.
This is both a bug and a feature request.
The current way to expand env variables in the Dex config is error-prone, since it happens before the YAML parsing.
Therefore, weird hacks are needed e.g. to support $ characters in the LDAP bindPW.
We're currently using a workaround there by defining an env variable ${DOLLAR} with value $.
#1051
#1382

Describe the solution you'd like to see
Add a toplevel config variable expandEnv which defaults to true (for compatibility with existing config files), but can be set to false by Dex admins who don't need variable expansion at all.

Describe alternatives you've considered
#1838

Additional context
I can make this change, but @sagikazarmark or @bonifaido can you tell me if such a PR would be accepted?

Current implementation:

These parts would be adapted in the PR.

func (s *Storage) UnmarshalJSON(b []byte) error {
	// ...
	if len(store.Config) != 0 {
		data := []byte(os.ExpandEnv(string(store.Config)))
		if err := json.Unmarshal(data, storageConfig); err != nil {
			return fmt.Errorf("parse storage config: %v", err)
		}
	}
	// ...
}

func (c *Connector) UnmarshalJSON(b []byte) error {
	// ...
	if len(conn.Config) != 0 {
		data := []byte(os.ExpandEnv(string(conn.Config)))
		if err := json.Unmarshal(data, connConfig); err != nil {
			return fmt.Errorf("parse connector config: %v", err)
		}
	}
	// ...
}

[sagikazarmark]

How about an env var instead that can disable this behavior? Then you don't have to parse the configuration upfront. Something like DEX_EXPAND_ENV.

The config layer needs a broader refactor in my opinion, but adding an env var like this would probably improve the situation in the short term.

[heidemn-faro]

@sagikazarmark an env variable is also fine for me. Then I'll implement it like this. 👍
DEX_EXPAND_ENV = 0 -> don't expand
any other value, or unset -> expand

[heidemn-faro]

Update:
DEX_EXPAND_ENV = falsy value -> don't expand.
Unset, or truthy value -> expand.