add linter and fix discovered issues

[okdas]

Summary

Adds golangci-lint and fixes discovered by linter issues.

Issue

• Foundational CI/CD in the shannon-sdk #2

Type of change

Select one or more:

• New feature, functionality or library
• Bug fix
• Code health or cleanup
• Documentation
• Other (specify)

Sanity Checklist

• I have tested my changes using the available tooling
• I have commented my code
• I have performed a self-review of my own code; both comments & source code
• I create and reference any new tickets, if applicable
• I have left TODOs throughout the codebase, if applicable

[okdas]

Issues discovered by linter and addressed in this PR. I kept the same configuration we have in the poktroll repo (but adopted for the newer version of the linter):

types/request.go:51:5: shadow: declaration of "err" shadows declaration at line 48 (govet)
        if err := proto.Unmarshal(requestBz, poktHTTPRequest); err != nil {
           ^
types/response.go:54:5: shadow: declaration of "err" shadows declaration at line 51 (govet)
        if err := proto.Unmarshal(responseBz, poktHTTPResponse); err != nil {
           ^
sdk/sdk.go:147:5: shadow: declaration of "err" shadows declaration at line 146 (govet)
        if err := sessionSupplierEndpoint.SessionHeader.ValidateBasic(); err != nil {
           ^
sdk/sdk.go:182:5: shadow: declaration of "err" shadows declaration at line 146 (govet)
        if err := relayResponse.Unmarshal(relayResponseBz); err != nil {
           ^
sdk/sdk.go:186:5: shadow: declaration of "err" shadows declaration at line 146 (govet)
        if err := relayResponse.ValidateBasic(); err != nil {
           ^
block_test.go:8:1: tests: ExampleLatestBlockHeight refers to unknown identifier: LatestBlockHeight (govet)
func ExampleLatestBlockHeight() {
^

[okdas]

Example of CI run when the linter is unhappy.

[red-0ne]

Awesome! Thanks @okdas