Introduce the concept of known EventGroup metadata types.

[SanjayVas]

Known EventGroup metadata types are types expected to be known by all callers. These need not be included in the EventGroupMetadataDescriptor FileDescriptorSet, and are instead loaded separately.

This also enforces uniqueness for EventGroupMetadataDescriptor message types. Note that this only affects the creation of new resources. Existing resources may exist that already violate this constraint.

Includes a workaround for bazelbuild/rules_proto#203.

Closes #1511

[wfa-reviewable]

This change is 

[stevenwarejones]

Reviewed 9 of 10 files at r1, all commit messages.
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[SanjayVas]

This was significantly reworked to account for the fact that some metadata message types should be shared. One example is SyntheticEventGroupSpec. PTAL.

Depends on world-federation-of-advertisers/common-jvm#235

CC @robinsons

[stevenwarejones]

What's the use case for sharing metadata messages?

Reviewed 19 of 59 files at r2, 1 of 3 files at r3, 2 of 2 files at r4, all commit messages.
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[SanjayVas]

1. We use shared metadata message types for the simulator test event groups
2. A given market may want all EventGroups across all EDPs to have a field with some common message type

Reviewable status: 29 of 68 files reviewed, all discussions resolved (waiting on @stevenwarejones)

[stevenwarejones]

2. Will this be the case? EDPs probably use different taxonomies and will need different event group metadata.

Reviewable status: 29 of 68 files reviewed, all discussions resolved

[SanjayVas]

A field with a common message type is not the whole message type. Basically the metadata can have a common part and an EDP-specific part. IIUC, having some commonality is a desire that has already been expressed by Origin.

Reviewable status: 29 of 68 files reviewed, all discussions resolved (waiting on @stevenwarejones)

[stevenwarejones]

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[stevenwarejones]

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[SanjayVas]

Reviewed 2 of 2 files at r6.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @SanjayVas)