Create the PlatformState protobuf

[poulok]

Create a protobuf definition for the PlatformState to include with one of the proposals for the Public Merkle API project.

There is an open PR in the protobuf repo to use as a starting point.

This effort needs to be coordinated with the TSS project because it will be affected. There are two options:

1. Do not wait for any changes to the PlatformState required by TSS. Include any changes later when they are known.
2. Delay this proposal until the TSS design is done and include any changes when they are ready.

The team has decided to follow option 1.

[imalygin]

hashgraph/hedera-protobufs#349