Add --upsert flag to update list commands

[CydeWeys]

Add --upsert (-u) flag to UpdatePremiumListCommand and UpdateReservedListCommand so that missing premium and reserved lists are created automatically when --upsert is specified, while still failing when --upsert is omitted and the list does not exist.

Allow CurrencyUnit to be nullable in PremiumListUtils.parseToPremiumList() so that PremiumList.createFromLine() infers the currency unit from the input lines when currency is unset, and use this in UpdatePremiumListCommand when creating a new list via --upsert. Also fix the mixed-currency example in config/files/premium/example.txt.

Also propagate exceptions from CreateOrUpdatePremiumListCommand.execute() and CreateOrUpdateReservedListCommand.execute() instead of catching and swallowing Throwable, and validate non-empty and parseable input data in CreatePremiumListCommand.prompt(). Add comprehensive unit tests across PremiumListUtilsTest and the create/update list command test suites.

BUG=http://b/569977604

---

This change is 

[CydeWeys]

I figured this approach was better than trying to make the shell scripts have to parse the text output of the create command determine if they needed to call the update command.

[gbrodman]

@gbrodman reviewed 13 files and all commit messages, and made 2 comments.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on CydeWeys).

---

core/src/main/java/google/registry/tools/CreatePremiumListCommand.java line 60 at r1 (raw file):

    }
    inputData = Files.readAllLines(inputFile, UTF_8);
    checkArgument(!inputData.isEmpty(), "New premium list data cannot be empty");

instead of checking here, as long as we're parsing the list anyway, we should parse the list then check to make sure there are at least some lines -- this would falsely succeed on a file with only a comment

---

core/src/main/java/google/registry/tools/UpdateReservedListCommand.java line 52 at r1 (raw file):

    List<String> allLines = Files.readAllLines(input, UTF_8);
    if (existingReservedList.isEmpty()) {
      newChange = true;

i think we also need to make sure the name is valid (though we cannot guarantee the TLD exists yet)