[WIP][FEA] Support Generalized Adjustment Criterion for Estimation plus Add Example Notebook

[nparent1]

Adds support for the generalized adjustment estimands at the estimation stage. The PR primarily does two things:

• Abstracts the get_backdoor() method of the IdentifiedEstimand class to instead be a get_adjustment_set() method. Then if the identifier_method is specified as 'general_adjustment', the get_adjustment_set() method will return the general adjustment set (rather than explicitly a backdoor set). Then throughout the repo where get_backdoor_set() was called for estimation purposes, we instead call get_adjustment_set(). Note that because we have get_adjustment_set() default to the backdoor set, the overall behavior is the same as before, unless a user explicitly sets general_adjustment as the identifier_method.
• Adds various tests and test utils. In particular, adds support for specifying explicit graphs in the estimation tests, for more specific tests of the general_adjustment criterion.

An example notebook is also added, dowhy_generalized_covariate_adjustment_estimation_example.ipynb

This is a follow-on PR to #1292

Issue Ref: #402