Feature Request: Add Validator Transactions

[faberto]

As the underlying functions are now in the web client nimiq/core-rs-albatross@069c5ae

I would like to propose an addition to the hub api to sign validator related transactions, especially

• create_validator
• update_validator
• retire_validator
• delete_validator

As it only needs to sign the transaction and not build it, the API doe not need a lot of options.
The kind and required information can be extracted and displayed:

Sample: