[Security] Add constants for access decision strategies

I suggest adding constants for the three access decision strategies (affirmative, consensus, unanimous).

They are difficult to spell, and without constants they are difficult to identify when reading the code.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A