Make this library multiplatform

[morki]

To make this library multiplatform, we need to replace this parts with multiplatform implementation:

• base64 encoding = kotlin stdlib
• current time = https://github.com/Kotlin/kotlinx-datetime
• cryptography = https://github.com/whyoleg/cryptography-kotlin
• http client (optional) = https://github.com/ktorio/ktor
• filesystem access (optional) = https://github.com/Kotlin/kotlinx-io

Blockers in https://github.com/whyoleg/cryptography-kotlin:

• [ ] support for ECDH (key agreement)
• [ ] support for HKDF (key derivation)

There will be some breaking changes, so it will be version 2.0 or another library (webpush-multiplatform).