[datalog] Add java entry map

[DeltaDizzy]

The C++ DataLogReader classes allow the user to get the name, type, metadata, etc. of any entry in a log given only its entry id using an internal map. The java library does not expose this functionality, so this is a proof of concept at one way this could be added. There are some very obvious warts but I want to get feedback if I am even barking up the right tree to begin with.

Closes #8126

TODO:

• Implement DataLogReaderRange usages
• Javadoc

[PeterJohnson]

The C++ DataLogReader class does not have these functions, only DataLogReaderThread does.

DataLogReaderThread reads the entire file (in the background) in order to build the map, and the user can separately check (or have a signal callback) to find out when reading is complete. The current approach in this PR has a major flaw in that it doesn't read the entire file for the user. A user would need to iterate over the entire file first (to build the map) before the added functions would work. This ordering dependency is not obvious in the API.

Also, in Java, you might not want to construct entries for the entire file, so this background scan should probably not do that, and manually decode only the start and finish records?