Cache SSLSocketFactories to allow okhttp HTTPS connection reuse (#1942)

* Reuse CustomCertManager

- Update bitfire-cert4android to 75cc6913fd
- Refactor HttpClientBuilder to use Optional for customTrustManager and customHostnameVerifier
- Add CustomCertManagerModule for dependency injection

* Implement connection security manager for HTTP client

- Introduce `ConnectionSecurityManager` and `ConnectionSecurityContext` classes
- Refactor `HttpClientBuilder` to use the new security manager for SSL context setup

* [WIP] Cache SSLContext by certificate alias

- Add context cache using Guava CacheBuilder
- Cache SSLContext in getContext method

* Update comments in HttpClientBuilder.kt for clarity

* Update ConnectionSecurityManager to use SSLSocketFactory caching

* Refactor socket factory caching logic for better clarity

* Add tests

* Refactor socket factory cache to store only SSLSocketFactory

* Minor changes
- Change socketFactoryCache to use LinkedHashMap instead of ConcurrentHashMap
- Update cache key handling to use String? instead of Optional<String>

* Add tests for caching

* Add logging

* Indenting

* Minor simplification

* Fix tests