Deserialization of RPC endpoint /abci_info fails with recent revisions of gaiad

[mzabaluev]

Version(s) of tendermint-rs: v0.24.0-pre.2

tendermint-rpc fails to parse the /abci_info response of gaiad in latest revisions from the gaia main branch.
Here is the relevant part of the error dump.

2: serde parse error
3: missing field `app_version` at line 10 column 5

The corresponding JSON serialization for the structure's fields in tendermint-go is annotated with the "omitempty" flag,
which likely causes the value to be omitted when the Go implementation considers it "empty". I have observed via a TCP dump that the app_version field is not present in the object.

Steps to reproduce

Encountered while trying to run integration tests for informalsystems/hermes#2213 with gaiad revision 67fbb896bfc46c32d4395892b149857fe27a7572

Definition of "done"

Allow all the fields in the JSON-RPC object to be absent, falling back to default values in the serde implementation for the corresponding tendermint-rpc data structure.