Extend MsappArchive APIs

[joem-msft]

This PR moves some new APIs used in the DocSvr code into the MsappArchive and IMsappArchive APIs in this repo.

• DoesEntryExist - allows callers to determine whether an entry exists w/o needing to inspect internal canonical dictionary.
• TryGenerateUniqueEntryPath - exposes logic for generating a unique entry, in case similar entries already exist. This function is similar to the private method GetSafeEntryPath, but this method only has a single responsibility, in order to make it more usable for different scenarios.
• MsappArchive.TryMakeSafeForEntryPathSegment is a new static method that has a single responsibility, allowing it's usage to be composed with other functions depending on the scenario. It is similar to encoding a path segment of a URI. Namely, the char set it allows is anything that could be a valid file name or directory name.
• GetSafeEntryPath is modified to use the new TryGenerateUniqueEntryPath and TryMakeSafeForEntryPathSegment methods.
• The GetEntry, GetRequiredEntry methods were updated to utilize the TryGetEntry method.

Validation:

• Added additional unit tests for the three new methods
• Updated existing test output due to '.' chars now being a valid file/directory name.

[mizrael]

do we really need to add these methods to the interface? can't we have them exposed as extension methods instead?

not needed