Add Generics for new benchmark components

[NickLucche]

Proposal/first draft addressing the type hint issue which we currently have with CLStream and custom CLExperience (language server can't see their properties and methods).
An example of this is the RLScenario which now properly displays the return type of its train/eval streams (RLExperience).

I've also added task labels for eval experiences when creating an RLScenario.

[coveralls]

Pull Request Test Coverage Report for Build 2271465330

• 8 of 35 (22.86%) changed or added relevant lines in 3 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.05%) to 70.599%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tests/benchmarks/scenarios/test_rl_scenario.py	0	9	0.0%
avalanche/benchmarks/scenarios/rl_scenario.py	2	20	10.0%

Files with Coverage Reduction	New Missed Lines	%
avalanche/benchmarks/scenarios/rl_scenario.py	1	28.36%
tests/benchmarks/scenarios/test_rl_scenario.py	1	18.18%

Totals
Change from base Build 2263070818:	-0.05%
Covered Lines:	12011
Relevant Lines:	17013

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💛 - Coveralls

[AntonioCarta]

Great!