Improve installation/configuration root detection.

[tobz]

Context

Currently, we depend exclusively on the configuration path passed to the ADP binary, along with environment variables, for configuring ADP. This includes the initial discovery of where the Core Agent (control plane) is running and how to connect to it.

While this is fine from a configuration standpoint, it depends on statically knowing where the binary is installed, in terms of potentially locating other relevant binaries/files -- IPC certificate, SGC (Datadog Secrets Backend), etc -- which breaks under the model of ADP running via Datadog Installer/Fleet Automation. DI/FA install the Agent (and thus ADP) in a version-specific directory alongside other versions, which means it's no longer enough to simply know that we look in path /var/foo when on Linux, but /opt/foo when on macOS, and so on.

We should add better support for detecting where ADP is running from, locating the installation root if possible, and using that as a primary factor in determining where the Agent/ADP are installed for extrapolating to other files, then falling back on platform-specific logic.