[Access] Implement GetScheduledTransaction* endpoints

[peterargue]

Implement GetScheduledTransaction and GetScheduledTransactionResult endpoints which allow querying transactions and results for scheduled transactions by their uint64 id.

This PR also makes the following changes:

1. Update GetTransaction, GetTransactionResult, and GetTransactionResultByIndex to allow querying scheduled and system transactions.
2. Update GetSystemTransaction and GetSystemTransactionResult to not allow querying for scheduled transactions since scheduled tx are not system tx.
3. Refactor the transactions endpoints to simplify the logic, which was needed to implement 1.
4. Rewrite unit tests to align with changes from 3.
5. Add functional tests using actual db implementations to test end-to-end flow.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 37.73036% with 642 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
access/unimplemented.go	0.00%	115 Missing ⚠️
engine/access/apiproxy/access_api_proxy.go	0.00%	70 Missing ⚠️
...pc/backend/transactions/provider/execution_node.go	0.00%	68 Missing ⚠️
engine/access/mock/access_api_client.go	0.00%	58 Missing ⚠️
access/mock/api.go	0.00%	44 Missing ⚠️
access/mock/transactions_api.go	0.00%	44 Missing ⚠️
engine/access/mock/access_api_server.go	0.00%	44 Missing ⚠️
...ne/access/rpc/backend/transactions/transactions.go	88.75%	31 Missing and 5 partials ⚠️
...transactions/provider/mock/transaction_provider.go	0.00%	27 Missing ⚠️
engine/access/rpc/handler.go	0.00%	26 Missing ⚠️
... and 14 more

📢 Thoughts on this report? Let us know!

[peterargue]

these tests were completely rewritten, so you can ignore the diff and focus on the new content

[peterargue]

Note: I took steps towards improving error docs and robust error handling, but didn't try to address everything. that will come as part of the optimistic sync API refactor.