feat: squash with prefix

[kamilsk]

Hi! What do you think about the possibility of adding prefix support while squashing structs?

Example:

type Bitbucket struct {
    Token config.Secret `mapstructure:"token"`
}

type GitHub struct {
    Token config.Secret `mapstructure:"token"`
}

type Service struct {
    Bitbucket `mapstructure:"bitbucket,squash"`
    GitHub    `mapstructure:"github,prefix"`
}

BITBUCKET_TOKEN=secret
GITHUB_TOKEN=secret

Now, I'm using the following workaround https://github.com/octomation/maintainer/blob/af46d826f7c070347b26fa46efee6dbdee4f8be3/internal/config/tool.go#L53-L64 but want to contribute to mapstructure the possibility to add prefix if squash it has.