Add stalebot config

[ricardogsilva]

Overview

This PR adds a github CI workflow file to manage stale issues and pull requests in accordance with RFC4.

It makes uses of the github stale action and is configured in such a way as to:

• Run every Sunday, at 03h01
• Mark issues and PRs that have been inactive for 90 days with the Stale stale label
• Close issues and PRs that have previously been marked as stale and have had no further activity for 7 days
• Exempts all issues and PRs that have the blocker label from being marked as stale

Notably, this PR sets the debug-only: true flag, which has the effect of simply performing a dry-run - the intention is that we would remove the debug flag after verifying if the dry-run outputs the expected results

Related issue / discussion

• RFC4
• Related to #1580

Additional information

Dependency policy (RFC2)

• [x] I have ensured that this PR meets RFC2 requirements

Updates to public demo

• [x] I have ensured that breaking changes to the pygeoapi master demo server have been addressed
  • [x] https://github.com/geopython/demo.pygeoapi.io/blob/master/services/pygeoapi_master/local.config.yml

Contributions and licensing

(as per https://github.com/geopython/pygeoapi/blob/master/CONTRIBUTING.md#contributions-and-licensing)

• [x] I'd like to contribute [feature X|bugfix Y|docs|something else] to pygeoapi. I confirm that my contributions to pygeoapi will be compatible with the pygeoapi license guidelines at the time of contribution
• [x] I have already previously agreed to the pygeoapi Contributions and Licensing Guidelines

[ricardogsilva]

cc @tomkralidis