Fix CI

[barrettj12]

Fix CI:

• Upload the charm to a temporary branch for testing - then cross-release this to edge on push.
• Add a separate CI job to select the Charmhub channels.
• Upload the charm in a separate job, outside of the integration tests.
• Rename the bootstrap job to integration.

Disable CI runs on pull_request:

• This fundamentally doesn't work as the CI workflow needs to access secrets to upload the charm to Charmhub - and GitHub doesn't allow pull_request runs to access secrets.
• For this reason, contributors should always open PRs from a branch on the main repo. Add a pr.yml workflow which checks this. (EDIT: I deleted this workflow cause it really does not seem to work at all. Something weird about the way GitHub triggers actions).

Enable CI on forks:

• Introduce a CHARM_NAME variable, which can be set on a fork to upload the charm to a different URL.
• Allow overriding the charm name in the charmcraft_tokengen.sh script, to allow contributors to easily generate the CHARMCRAFT_AUTH secret for their forks

  CHARM_NAME=barrettj12-cc .github/charmcraft-tokengen.sh
  

QA steps

• GitHub Actions should pass on this PR.
• GitHub Actions should pass when run on a fork (here).
• The new PR workflow should fail when run on a PR opened from a fork. See Test PR workflow #47
• Automatic edge releases work:

  $ juju info barrettj12-cc
  latest/edge:       65  2023-10-18  (65)  10MB

Links

Jira card: JUJU-4663