feat: add last blocktime endpoint

[dongsam]

Description

Add /mint/v1beta1/last_block_time endpoint to get last_block_time by grpc
The mint module stores last_block_time in blocks for inflation calculation but has not provided endpoints so far. Currently, in order to obtain a specific block time, a call must be made by rpc, but it can be obtained through grpc query by opening the corresponding endpoint.

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[hallazzang]

I'm wondering why the last block time should also be exposed through gRPC.

[dongsam]

I'm wondering why the last block time should also be exposed through gRPC.

@hallazzang

As an example, we need to know the block time of every block to calculate the uptime in market maker scoring system, where it is much more efficient to call the only last block time with gRPC than to get the whole block information through tendermint rpc and parse the block time

[hallazzang]

LGTM 👍