Clickhouse peer: specify S3 fields

[Amogh-Bharadwaj]

This PR refactors the Clickhouse peer to now have the S3 fields - object path, access key id, secret key and region:

message ClickhouseConfig{
  string host = 1;
  uint32 port = 2;
  string user = 3;
  string password = 4;
  string database = 5;
  string s3_path = 6; // path to S3 bucket which will store avro files
  string access_key_id = 7;
  string secret_access_key = 8;
  string region = 9;
}

The UI for Clickhouse peer creation has been updated accordingly:

Validation for the S3 stage is also added as part of validate peer for clickhouse.

Nexus code updated accordingly.

This PR has been functionally tested by validating and creating a clickhouse peer followed by performing a basic CDC PG -> CH mirror