Make Sprig functions available in config template

[croissong]

This change makes all functions from the Sprig template library available in the manifest.

Coming from Helm/Helmfile, I was missing the various utility functions provided by Sprig while building my config.
For example, my personal config I am iterating over a packages map and would like to merge each package config with a default (showing the use of mergeOverwrite from Sprig):

# values.yml

defaults:
  trimPrefix: true

packages:
  sheldon:
    owner: rossmacarthur
    trimPrefix: false

  sttr:
    owner: abhimanyu003

  updatecli:
    owner: updatecli

# updatecli.yaml

sources:
  {{ range $name, $package := . packages }}

  # set defaults
  {{ $package = mergeOverwrite .defaults $package }}
  ...

Test

I slightly refactored pkg/core/config/template.go to be able to mock the filesystem interactions, and I added some basic tests for the templating functionality.

Docs

I created updatecli/website#672 to mention Sprig in the docs.

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[olblak]

Waouw that's something I wanted so much \o/
Let me run some tests

[olblak]

Thanks @croissong for the pullrequest. It looks great. Once merged I'll take a few days to test the main branch, before releasing 0.45 as I did a few changes around git operation which may introduce regression and I want to be extra safe