feat: switch from `&str` params to `String`

[FrenchGithubUser]

Description

Switched &str in struct attributes to String. We will be using this library for our push notification gateway Hewdig, and those changes would simplify it use, as well as help future users of a2.

Resolves https://github.com/reown-com/a2/issues/67

How Has This Been Tested?

cargo test

Due Dilligence

• [x] Breaking change
• [x] Requires a documentation update
• [ ] Requires a e2e/integration test update

[FrenchGithubUser]

This should be ready for review @geekbrother :)