Support generated secrets for templates

[bohdan-shulha]

For example, APP_KEY for laravel.

It has the next format:

APP_KEY=base64:Z7CI/ANMYiIBKT+KQFAFC+AuMZ2dwWhwzwaRNFJ/CBE=

The proposed format (an example):

                        {
                            "name": "APP_KEY",
                            "value": "base64:{-randomBytes:32|base64-}"
                        },

(for APP_KEY are following requirements)

 Unsupported cipher or incorrect key length. Supported ciphers are: aes-128-cbc, aes-256-cbc, aes-128-gcm, aes-256-gcm. 

The syntax {- ... -} has been selected to distinguish the dynamic expressions with the static vars embeddings (i.e. {{user}})

In the scope of this ticket, need to add the following expressions:

1. {-username-}: should be generated from the common words (see the Service slug generation example), words separated with underscore _
2. {-password-}: random string of 32 chars containing ascii symbols (A-z0-9 and special chars)
3. {-randomBytes:32|base64-}: expression from the example; self-descriptive.

These expressions have to be processed server-side to enable their use not only in app templates, but also in custom services defined in-place by customers.

The "use template" form should allow users to toggle between "simple" and "advanced" views. When displaying a simple view, only the fields which require user's input should be displayed. The advanced form should be displaying all fields, including those with the expressions.

TBD: mb instead of username would be better to create some generic equivalent like phrase:words,2|slug (but usernames are so common, so I'd prefer having the shortcut).