Add optional flags to provides endpoints

[rpbritton]

Issue

Since provides endpoints can be non-optional, can the default value of optional is false, these endpoints are assumed non-optional.

Solution

Set optional flag on provides endpoint to indicate correct optionality.

Checklist

• I have added or updated any relevant documentation.
• I have cleaned any remaining cloud resources from my accounts.

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[paulomach]

Hi @rpbritton, in principle there's nothing wrong, but can you explain the concept/motivation here?

[paulomach]

@rpbritton also, please rebase to pick fix in tests

[rpbritton]

Hi @rpbritton, in principle there's nothing wrong, but can you explain the concept/motivation here?

Hi @paulomach , the context here is for automated Charm testing we need to deduce the minimal deployment using these flags for the test (so they should reflect which integrations are required to successfully deploy the charm, a neighboring charm, and test the integration between them).