Add ability to get expiration information

[Shaptic]

Is your feature request related to a problem? Please describe.
There's no obvious way to get the expiration ledger anymore.

Describe the solution you'd like
Server.getLedgerEntries et al. should allow a fetchExpiration parameter that will automatically include a fetch for the corresponding expiration ledger entry.

Describe alternatives you've considered
You can do this manually, but you need to know oddly-specific internals about Stellar to achieve it.

This could be done on the RPC side, but it may take longer and this can be a stop-gap.

Additional context
Workaround explanation.
Discord context.