add dependabot to the project

[skarzi]

Dependabot automatically bump your dependencies.

References:

• https://docs.github.com/en/code-security/supply-chain-security/keeping-your-dependencies-updated-automatically/configuration-options-for-dependency-updates

[skarzi]

Sample dependabot configuration file: https://github.com/skarzi/drf-exception-dispatcher/blob/main/.github/dependabot.yml