Feat[bmq]: support exists(x) function in subscription expressions

[bbpetukhov]

Issue number of the reported bug or feature request: #<219>

Describe your changes

• Added new function exists(x) to check if the variable x is defined in the current context

Testing performed

• Added various unit tests to cover exists(x)

[678098]

@bbpetukhov @chrisbeard some time ago, we had an idea of declaring internal properties or keywords with two underscores __, like __timestamp. Do you think we should do this? Should we also make checks when declaring properties for messages, to prevent setting properties with names abs or exists?

[bbpetukhov]

@bbpetukhov @chrisbeard some time ago, we had an idea of declaring internal properties or keywords with two underscores __, like __timestamp. Do you think we should do this? Should we also make checks when declaring properties for messages, to prevent setting properties with names abs or exists?

In the latest version of this PR the implementation of "exists" keyword is smart enough to support both use cases: as a property and as the built-in function. So there should not be any conflicts with property names. Unit tests are added for this scenario.