[breaking change] Do not use Mustache but SpringBoot like references to refer to secrets and globals

[eolivelli]

Problems statement:

1. We using Mustache both for resolving variables (secrets and globals) and for templating of Prompts and Queries.
   This nesting of Mustache templates is really awkward and the use must use a strange '{{%' syntax in Prompts and Queries.
2. Mustache only produces strings, and it cannot return maps or lists, so you cannot use global variables for lists or maps
3. you must use the triple curly braces in order to not escape the values ({{{secrets.password}}} works, {{secrets.password}} sometimes doesn't work)

Modifications:

• Drop Mustache from the preprocessor, let's use a StringBoot like syntax ${secrets.x.y.z} and ${globals.x.y.z}
• Change all the examples to use the new syntax

[nicoloboschi]

this is unrelated

[eolivelli]

fixed

[nicoloboschi]

unrelated

[eolivelli]

fixed