fix: add StringAdapter for Enforcer to pass policy as string

[cs1137195420]

fix: #200

Description

I added void LoadPolicy(std::string policy) for Enforcer to pass policy to Adapter as a sting buffer, which could be called as the following code:

casbin::Enforcer e("./path/to/model.conf");
std::string policy = "p, alice, data1, read\np, bob, data2, write";
e.LoadPolicy(policy);

Since c++ doesn't support overloaded functions with the same number and type of parameters, I didn't add a constructor like Enforcer(const std::string& model_path, const std::string &policy) in issue #200, that conflicts with Enforcer(const std::string& model_path, const std::string& policy_file)

[casbin-bot]

@EmperorYP7 @divy9881 @noob20000405 @sheny1xuan please review

[hsluoyz]

@cs1137195420 fix all errors:

[cs1137195420]

@hsluoyz I modified the code referring to the implementation of StringAdapter in golang. Is this PR OK now?

[hsluoyz]

@sheny1xuan plz review

[cs1137195420]

@sheny1xuan I've modified the code, could you plz review it again?

[hsluoyz]

@MaximAxelrodFirebolt plz review

[github-actions[bot]]

:tada: This PR is included in version 1.50.2 :tada:

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