Dynamic configuration expression support

[foxcpp]

1. Define evaluation context that is passed across the codebase via context.Context.
2. Replace some (most?) configuration values with generic dynamic like:

type DynamicString struct {
	...
}

func (ds DynamicString) Evaluate(ctx context.Context) (string, error)

3. Add support for single-quote escaping in maddy configuration syntax.

modify {
  replace_rcpt expr 'address_username(key)'
}

4. Define configuration syntax (like above) for dynamic expressions.

[foxcpp]

Possible use for #731

destination_in expr 'address_domain(recipient) in table_query(local_domains, '')'