This Minecraft data pack allows you to enchant a player's hands with silk touch, allowing them to obtain (almost) any block they break. More details on Planet Minecraft.
Go to the releases page.
Each version states the compatible Minecraft versions.
Choose the latest version that is compatible with your desired Minecraft version.
From the list of assets, download the first file, named something like Silky-Hands-2.4.zip.
Add it to your world like any other data pack.
If you don't know how to add the data pack to your world,
here is a tutorial.