feat: Rewrite the Tensorboards Web App charm following the sidecar pattern

[orfeas-k]

This PR rewrites the Tensorboards Web App (TWA) charm following the sidecar pattern In short:

• Update pip packages' versions (bumped SDI version in TWA charm)
• Update README.md file to match the charm where it is located
• Introduce CONTRIBUTING.md file
• Add upstream manifests as Jinja2 templates
• Set charm's status to blocked when there is no ingress relation
• Rewrite the charm following the sidecar pattern
• Adjust joint charms' deploy test to match changes
• Add integration tests for TWA charm
• Run TWA integration tests in Github CI
• Drop TWA container port from config option
• Use production as default for config's Backend-mode

Upgrade instructions

1. Trust the charm

   juju trust tensorboards-web-app

2. Refresh the charm providing: i. the image since oci-image was changed to tensorboards-web-app-image. ii. the path to the local charm or the corresponding channel (for charms released in charmhub)

   juju refresh tensorboards-web-app --resource tensorboards-web-app-image=kubeflownotebookswg/tensorboards-web-app:v1.7.0 --path="./tensorboards-web-app_ubuntu-version-arch.charm" 

   or

   juju refresh tensorboards-web-app --resource tensorboards-web-app-image=kubeflownotebookswg/tensorboards-web-app:v1.7.0 --channel=track/risk

[ca-scribner]

Extremely pedantic suggestion for the upgrade instructions :)

juju refresh tensorboards-web-app ... --channel=version/mode

should be

juju refresh tensorboards-web-app ... --channel=track/risk

Doesn't change the function, but gets the terminology consistent with elsewhere.