Future proof options for mithril client

[jpraynaud]

Why

The Mithril client needs to be configurable in a future proof way: we don't want to update the API surface whenever a new option is needed. For example, in #1720, we have expressed the need to add a custom HTTP header.

What

Implement an optional mechanism for the Mithril client and provide a first option for custom HTTP Headers.
In order to implement a stable mechanism we will represent the options as a HashMap<OptionName, OptionValue> where OptionName is a String and OptionValue is an Enum.

How

• Implement a new client constructor in WASM which has a new parameter for options of the client ,or modify the current constructor to include a new optional parameter
• Have the client builder use the options to build the client
• Update the explorer
• Update the doc and examples (+ make an example with additional headers)
• Announce breaking change:
  • in the CHANGELOG
  • in the next distribution

Related issues

#1720