Add sanitization for project_root in agent configuration

[sanketsaurav]

The agent must be configured with the absolute path of the project repository, and the value must be validated before setting. This is important because all source-code usage inference depends on the file location, for which we must know the base path of the application.

Presently, the project_root is directly being set after the user.

beacon-py/beacon/agent.py

Lines 150 to 152 in e29df95

	def _set_project_root(self, project_root):
	# TODO: sanitize project root with defaults
	self.project_root = project_root

This has to be handled better.