Add config option for log level (#2391)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2391

Add a buck config option to set the ExecuTorch log level. Used as follows:

`buck2 run ... -c executorch.log_level=[level]`

Where [level] is one of Debug, Info, Error, or Fatal (case-in-sensitive).

Log methods are refactored into a separate bazel file to facilitate re-use. I need to override the XNNPACK log level when ET log level is set to Debug, and I don't want to duplicate the log level parsing logic.

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Reviewed By: digantdesai

Differential Revision: D54800053

fbshipit-source-id: f66e8524343d594c099cc864e6ad4d675ae19a3c