chore: introduce typed context keys

[xlgmokha]

What kind of change does this PR introduce?

Refactoring. This change adds a typed context key that can be used to add an item to context and fetch it safely.

What is the current behavior?

Each read from context.Context with an unchecked type assertion can panics if the value is the wrong type.

Before:

var tokenKey = contextKey("jwt")

func withToken(ctx context.Context, token *jwt.Token) context.Context {
  return context.WithValue(ctx, tokenKey, token)
}

func getToken(ctx context.Context) *jwt.Token {
  obj := ctx.Value(tokenKey)
  if obj == nil {
    return nil
  }
  return obj.(*jwt.Token)
}

What is the new behavior?

After:

var tokenKey = ctxkey.New[*jwt.Token]("jwt")

func withToken(ctx context.Context, token *jwt.Token) context.Context {
  return tokenKey.WithValue(ctx, token)
}

func getToken(ctx context.Context) *jwt.Token {
  return tokenKey.Value(ctx)
}

Additional context