Add feature flag on account_changes and fungible_token_events

[eduohe]

Closes #346

[telezhnaya]

Great job, thank you!
Could you please add the comment to readme about this, with the instruction how to enable this functionality?
We will add the link to the announcement later when we post it, but anyway we need to say at least 1 sentence here what's going on

[khorolets]

Overall LGTM! Although, I don't like the naming you've chosen. How about making it a bit shorter:

• ft_events
• account_changes

I propose dropping the load_ stuff as I haven't seen such naming in Rust features. @telezhnaya what do you think? I don't insist.

[telezhnaya]

I don't have the strong opinion about the naming, we can go with any option.
I'm OK with dropping load_ prefix, but I personally prefer leaving fungible_token_events to match the naming we use in the code right now.

[telezhnaya]

Major:
If we want to deploy this to production (we really want to), we need to rewrite start-from-interruption logic as well
The easiest way is to copy-paste the solution from microindexers: https://github.com/near/near-microindexers/blob/main/indexer-opts/src/lib.rs#L79

[eduohe]

Thanks @telezhnaya and @khorolets , suggestions implemented

[telezhnaya]

Please run cargo fmt & cargo clippy