Report the version from the elastic agent package instead of agent binary

[joshdover]

Issue

We need to be able to repackage Agent without rebuilding the Agent binary, and still have Agent report the updated version number at runtime. The repackaging step will be created as part of #2498

This requires that agent reads its version from a plaintext file that can be updated as part of the packaging step, rather than written directly into the binary.

Suggested solution

Implement after #2579 to use the full version number contains in the parent directory.

Definition of done

• The Agent is reporting the correct version and not a fixed one from building steps.
• Tests are in place to confirm that the full version is the one reported.

[cmacknz]

This requires that agent reads its version from a plaintext file that can be updated as part of the packaging step, rather than written directly into the binary.

Depending on what we do in #2579 (comment), another option could be for the agent to report the version number from its parent directory e.g. elastic-agent-7ac68h-8.8.1.er1.

[jlind23]

Moving this to a later sprint as the plan changed a bit.

[joshdover]

We are moving this back into the sprint. cc @pchila

[pchila]

Since the version command is usable even without the agent being installed I am going with the solution of a file being created and packaged in the agent (similar to what we already do with the build_hash.txt.
As for the version format I am going with the same structure as we use for the agent version:
elastic-agent-x.y.z[-SNAPSHOT]+<commithash> which differs from the released version x.y.z[-SNAPSHOT]+<buildid> (the part after the + is a different identifier.
Is that acceptable ?

[cmacknz]

elastic-agent-x.y.z[-SNAPSHOT]+ which differs from the released version x.y.z[-SNAPSHOT]+ (the part after the + is a different identifier.

Yes, using instead of seems preferable here, since the buildid is hard to tie back to a specific set of commits on its own.