Support entity access policies

[foundrytom]

What

Allow the BAL library to define that specific entities are read-only/write only and disallow publishing

Why

Allows that branch of OpenAssetIO api contracts to be tested

ACs

• By default, resolve and entityTraits treats all access modes the same as the current happy path (i.e. read).
• Exceptions can be added for explicit access modes that override the data in the versions list. This applies to all entity trait queries.
• An exception for an access mode that maps to null disables that access mode, with appropriate errors raised.
• Publishing methods (preflight and register) error based on management policy.
• Publishing methods (preflight and register) error based on entity traits with write policy.
• Update https://github.com/OpenAssetIO/OpenAssetIO-MediaCreation/pull/69

[foundrytom]

The main idea here is that making existing entities read-only per-se is prohibitive, and not a pattern we want to encourage (imaging opening a work file, making a change, then republishing). If you couldn't just republish to the existing entity, you'd need some overly complex getWithRelationship dance.

[feltech]

Note that a prior simple addition to support access modes in resolve #98 has a bug, in that supported_access_modes does not propagate to newly published versions, so once an entity is published it becomes kRead-only.

It's likely this existing mechanism will be rewritten as part of this issue, so not creating a separate bug report.