Move `openassetio` dependency to extras

[foundrytom]

What

Remove hard dependency on openassetio and move to an extras

Why

openassetio should always be provided by the host runtime (to ensure we don't have multiple copies loaded simultaneously). Having it as a hard dependency anywhere up the stack causes problems, as pip will try and provision it too, which usually results in a separate static build being installed in addition to whatever the host provides (unless the host is pure python).

ACs

• openassetio moved to an extras dep under the runtime name.

[foundrytom]

Closing in favour of #58