Added support for selecting schema to be exported in blocks and transactions

[Haaroon]

Added support for selecting schema to be exported in blocks and transactions in the cli.

If a user prefers to export certain fields of the block/transaction schema they can by added the fields to be exported as a list into the cli.

This list is only compatible with the schema in the ethereumetl/jobs/exporters/blocks_and_transactions_item_exporter.py file.

If an empty schema is given, then the entire schema is exported as usual. Schema with spelling mistakes and invalid fields are ignored.

Apologies I have unable to write tests at this time.

[medvedev1088]

Thank you! I will try to review it this week.

[btfdip]

this feature incorporated ?

[Haaroon]

this feature incorporated ?

@btfdip no it wasnt implemented :(