Create an extension for parsers

[ericwb]

Is your feature request related to a problem? Please describe.
N/A

Describe the solution you'd like
Bandit currently utilizes the Python base ast module for parsing. But there are other
parsers out there that might be beneficial to use instead. For example, libcst is concrete
syntax tree, which may provide additional capabilities to analyze the code more deeply or
provide better remediation. Another parser is tree-sitter which could be more performant and potentially add the capability to parse other languages.

The feature being proposed here is to provide a extension mechanism similar to what we
already provide for formatters and check plugins. Then abstract the hard-coded calls to the
ast module to use the extension manager.

Describe alternatives you've considered
If someone desires to utilize a different parsing mechanism, the only other option
would be to rewrite much of the guts of Bandit with the new parser.

Additional context
https://libcst.readthedocs.io/en/latest/index.html
https://tree-sitter.github.io/tree-sitter/

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