[Elastic Agent] Provide the ability for the user to set the name of their agent during installation

**Use Case:** Currently the name attached to the agent is chosen by Elastic - as in derived from some other entity (i.e machine name). Often these names have no significance to the user of our product who many need to better visually differentiate. In some instances the user is operating a multi tenanted environment and would like to encode the tenant name/ID into the agent name.

**Proposal:** Provide a flag for the user to overwrite the given name during installation/enrollment. This name should appear in the Fleet UI.

**Suggested syntax:**

elastic-agent enroll --url=https:// --enrollment-token= --name=”CEO laptop”

elastic-agent install --url=https:// --enrollment-token= --name=”CEO laptop”

Start with the elastic-agent enroll and install entry points described in the issue, then trace how the agent name is currently derived and sent for display. Check both installation paths and verify that the supplied --name value appears in Fleet UI while the existing default remains unchanged when the flag is omitted.

Written by the indexing model from the issue text.