Add memo field for file entity

[donaldthibeau]

Summary
Each file entity should include an optional memo field of up to 100 bytes. Uniqueness is not guaranteed. The memo can only be changed by the admin keys on the file entity.

Memo field would be returned as part of file info.

[donaldthibeau]

So to summarize, for an entity type of X we will say that for the CreateX transaction:
• TransactionBody contains a memo describing the create transaction. This goes in the receipt (and so in the record)
• XBody contains a memo that will be copied to the new entity
• If X is Schedule then the scheduled transaction will contain its own memo (or 2 memos if it is a Create transaction) which will have the described effects when it executes. (A CreateScheduled cannot contain another CreateScheduled, so there won't be further nested memos)
All the above is also true for an UpdateX transaction.
If that's the plan, then everything is current ok, except:
• TransactionBody should have a better comment (specifying length in bytes)
• CreateAccount needs another memo, to be similar to other Create transactions.
NOTE: Any CreateX transaction should have memos defined as a protobuf string, but any UpdateX transaction should define the memo as a StringValue. That is needed so that we can tell the difference between leaving it out (and so not changing it) vs. putting it in with an empty string (and so changing it to the empty string).