feat: add linux install script for manual installation support

[obs-gh-mattcotter]

Description

Add linux install script for manual installation support. This can install from a local zip or from our release downloads.
Examples:

$ bash ./scripts/install_linux.sh 
Downloading latest release from GitHub
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
100 34.0M  100 34.0M    0     0  42.6M      0 --:--:-- --:--:-- --:--:-- 42.6M
Creating system install folders. This may ask for your password...
Leaving existing observe-agent.yaml in place.
Observe agent successfully installed to /etc/observe-agent

$ observe-agent version
Using config file: /etc/observe-agent/observe-agent.yaml
observe-agent version: 1.4.0

$ ZIP_DIR=dist/linux_amd64_v1/observe-agent_Linux_x86_64.tar.gz bash ./scripts/install_linux.sh 
Installing from provided zip file: dist/linux_amd64_v1/observe-agent_Linux_x86_64.tar.gz
Creating system install folders. This may ask for your password...
Leaving existing observe-agent.yaml in place.
Observe agent successfully installed to /etc/observe-agent

$ observe-agent version
observe-agent version: 1.4.0-SNAPSHOT-48bafce6
observe-agent config file: /etc/observe-agent/observe-agent.yaml

Checklist

• Created tests which fail without the change (if possible)
• Extended the README / documentation, if necessary

[obs-gh-alexlew]

do we want to set up the systemctl service file here too? for parity with the actual apt install