Replace mapstructure with fork

[adriansmares]

Summary

Closes #5343
References mitchellh/mapstructure#278

Changes

• Replace mapstructure with a patched fork which correctly handles typed nil values

Testing

Unit testing + testing with the following diff:

diff --git a/cmd/ttn-lw-stack/commands/root.go b/cmd/ttn-lw-stack/commands/root.go
index 0064f8e60..901f08660 100644
--- a/cmd/ttn-lw-stack/commands/root.go
+++ b/cmd/ttn-lw-stack/commands/root.go
@@ -17,6 +17,7 @@ package commands
 
 import (
        "context"
+       "fmt"
        "os"
        "time"
 
@@ -70,6 +71,8 @@ var (
                                return err
                        }
 
+                       panic(fmt.Sprintf("%v", config.NS.DefaultMACSettings.DesiredADRAckDelayExponent))
+
                        // enable configured experimental features
                        experimental.EnableFeatures(config.Experimental.Features...)
 

Before this PR the value would be a zero value instead of nil (even though the default is nil):

panic: ADR_ACK_DELAY_1

goroutine 1 [running]:
go.thethings.network/lorawan-stack/v3/cmd/ttn-lw-stack/commands.glob..func16(0x42def20?, {0x26b188a?, 0x1?, 0x1?})
        /home/adriansmares/go/src/go.thethings.network/lorawan-stack/cmd/ttn-lw-stack/commands/root.go:74 +0xe8
github.com/spf13/cobra.(*Command).execute(0x42def20, {0xc000e7e9b0, 0x1, 0x1})
        /home/adriansmares/go/pkg/mod/github.com/spf13/cobra@v1.2.1/command.go:835 +0x58b
github.com/spf13/cobra.(*Command).ExecuteC(0x42deca0)
        /home/adriansmares/go/pkg/mod/github.com/spf13/cobra@v1.2.1/command.go:974 +0x3b4
main.main()
        /home/adriansmares/go/src/go.thethings.network/lorawan-stack/cmd/ttn-lw-stack/main.go:26 +0x25
exit status 2

Post PR:

panic: <nil>

goroutine 1 [running]:
go.thethings.network/lorawan-stack/v3/cmd/ttn-lw-stack/commands.glob..func16(0x42def60?, {0x26b188a?, 0x1?, 0x1?})
        /home/adriansmares/go/src/go.thethings.network/lorawan-stack/cmd/ttn-lw-stack/commands/root.go:74 +0xe8
github.com/spf13/cobra.(*Command).execute(0x42def60, {0xc000561370, 0x1, 0x1})
        /home/adriansmares/go/pkg/mod/github.com/spf13/cobra@v1.2.1/command.go:835 +0x58b
github.com/spf13/cobra.(*Command).ExecuteC(0x42dece0)
        /home/adriansmares/go/pkg/mod/github.com/spf13/cobra@v1.2.1/command.go:974 +0x3b4
main.main()
        /home/adriansmares/go/src/go.thethings.network/lorawan-stack/cmd/ttn-lw-stack/main.go:26 +0x25
exit status 2

Regressions

This was already broken.

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI, according to the compatibility commitments in README.md for the chosen target branch.
• Documentation: Relevant documentation is added or updated.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.