Support tx-indexer provider

[jinoosss]

tm2-js-client needs to support indexer queries for some additional extension support.

The getGasPrice function gets information from the indexer for implementation and additional feature support.

The current Provider is a different type than the indexer Provider and must be injected and used separately in the wallet instance (e.g. TM2RPCProvider, IndexerProvider, etc.).

We need to discuss the scope of support for indexers.
Indexers can be queried with RPC methods or GraphQL.

[jinoosss]

@zivkovicmilos ,
I'd love to know what you think.