Using custom CA certificates

### Please agree to the following

- [x] I have searched [existing issues](https://github.com/cryptomator/android/issues?q=) for duplicates
- [x] I agree to follow this project's [Code of Conduct](https://github.com/cryptomator/android/blob/develop/.github/CODE_OF_CONDUCT.md)

### Summary

Implementing the ability to use custom CA certificates for webdav

### Motivation

When I connect to my server over webdav-https, I am asked if I trust the certificate. I do in my case and normally the root CA, which is my custom certificate, is already stored in the android certificate manager. Thus, it seems what is missing is a way to automatically compare TLS certificates against custom root certificates.

This can for example be done by somehow accessing the system CA certificates or by for example importing a CA file when setting up a webdav connection.

### Considered Alternatives

_No response_

### Anything else?

_No response_

Start with the WebDAV-HTTPS connection setup and Android certificate manager behavior described in the issue. Compare using system CA certificates with importing a CA file, then define the supported configuration and verify that a WebDAV connection using a trusted custom root certificate succeeds without an unwanted certificate prompt.

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