Adds installation information to Agent's flare

[mx-psi]

What does this PR do?

Adds a new file to the Agent's flare that will contain metadata about agent installation.

Motivation

• Allow users to track their installation methods
• Allows Datadog to make stats on installation methods

Additional Notes

The install_info file added to the flare by this PR is not yet created by the different installation methods, so it should be manually created at the Agent's configuration directory for testing.

Describe your test plan

• Create a mock install_info file at the Agent's configuration directory
• Run the agent flare command to create a flare and check that a copy of the install_info file is available in the resulting zip.
• Delete the mock file and repeat the process to see that the flare is created correctly without that file. Ensure that the missing file error is logged.