Support map literals

Currently it is not possible to create map literals (consisting of several key/value pairs) in HTL expression language. This would be especially handy for passing request attributes (see https://issues.apache.org/jira/browse/SLING-5812) which may now be given to `data-sly-include`. Currently one needs to have some backing code in JavaScript or Java to be able to use that option.

Maybe JSON notation would work
```
${'key1':'value1','key2':'value2'}
```

No implementation file or test is named. Start by reviewing the HTL expression language specification and the linked SLING-5812 request, then define map-literal syntax and its support for passing request attributes; done means the specification covers multi-pair map literals and their use with data-sly-include.

Written by the indexing model from the issue text.