Update icon for Fleet Server and Elastic Agent

[mostlyjason]

Right now we just have the default icon of a box for Fleet Server. Can we pick an icon that better reflects its function? To clarify, it should be an icon, not a logo.

Update 2021-07-29 We have icons now for Fleet Server and Elastic Agent

CC @hbharding

[mostlyjason]

@jen-huang should fleet team take this one since we are updating the dashboard for elastic agent anyway?

[nchaulet]

@mostlyjason I am about to update the dashboard for elastic agent, if you know where I can find the assets I can update the icons too.

[mostlyjason]

Here is the file https://github.com/elastic/Design/issues/2484#issuecomment-880978454. I think @jen-huang is also adding it to EUI

[jen-huang]

@nchaulet Here's the associated EUI ticket elastic/eui#4981