token: Add extension support in mint init

[joncinque]

Problem

People want to initialize mints with extensions

Solution

Add that ability. A few notes from this PR:

• introduces StateWithExtensionsOwned, used in testing and client situations, where the TLV data is owned instead of borrowed. There's some copy-pasta because vec uses split_off and slice uses split_at
• can initialize a mint close authority
• initialize mint supports extensions
• ExtensionInitializationParams for cleaner init!

Edit: Reduced the scope of this to just initialization and tests. The actual support for the mint close authority will come in a separate PR