refactoring and changes to how customizations work

[dan-v]

• new: add support for custom config repos and deprecate old customization methods (custom-patches, custom-manifest-remotes, custom-manifest-projects, custom-prebuilts)
• new: also add support for core config repos that can be used to build an OS with plain AOSP rather than using the default RattlesnakeOS core. closes Allow custom build manifests #129.
• new: add support for Pixel 4a 5G. closes Support for Pixel 4a 5G Bramble #180.
• deprecate: encrypted-keys option (for now) to reduce build script complexity
• deprecate: hosts-file (can be added in custom config repo)
• move version checking to lambda function and base everything on a 'release' now rather than individual components, and add version pinning of core components. closes Move update checks into Lambda function #47.
• move templates out into individual files with new go embed functionality and move rendering of these to it's own function
• start breaking apart shell script into generic raw shell script (that can be shellchecked) and the more dynamic pieces that would change across clouds
• have lambda function poll and wait for a spot instance to launch rather than immediately returning success
• change from building fdroid to using prebuilt and use the default privileged extensions without any changes
• add an option to disable building Chromium (should provide your own webview with customizations in this case)
• add option to disable cron by setting schedule to empty string
• for deploy command add --skip-deploy flag that will just render the files to disk
• automatically backup config file to s3 bucket
• move to using gp3 storage for build instances and bump to 300gb of storage
• cleanup makefile, add shellcheck, add tests
• update readme, add new config config methods, removed deprecated options
• upgrade to ubuntu 20.04 AMI
• move to go 1.16
• move to aws go v2 sdk