Move dependencies to extras

What

Remove hard dependency on openassetio and openassetio-mediacreation and move to 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 a extras dep under the all and deps names
• openassetio-mediacreation moved to an extras dep under the deps name. [TBC]

首先定位软件包依赖配置，并检查现有的 openassetio 和 openassetio-mediacreation requirements 是如何声明的。确认 openassetio 可通过 all 和 deps extras 获取，并确认 openassetio-mediacreation 可通过 deps 获取；在将工作视为完成之前，解决已指出的不确定性。