Support variables, references to members, late-initialization

[fred-labs]

Until now the final parameters of actions were generated during parsing.
To be able to support additional use cases, it is required to resolve just before execution.
With this PR the parsing is moved to a newly defined execute() method that is called with the actual execution of the behavior.

One use case is, that an action might change variables values for later usage. This is now possible by internally creating a py-trees blackboard variable for any VariableDeclaration.

Additionally it is now supported to refer to members of parameters (that can either be fixed parameters or variables). Example:

struct test_struct:
    member: string = "foo"

scenario test_scenario:
    do serial:
        log(test_struct.member)

[github-actions]

Test Results

235 tests   221 ✅  27m 29s ⏱️
 14 suites   14 💤
 14 files      0 ❌

Results for commit 18474ce.

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