A more secure and generalized approach for PDO Basic Auth Backend

[lightbluetom]

Hello,

motivated by the currently insecure password hashing in sabre/dav (as discussed in Baikal #514), I developed this PR. It allows administrators to choose any password hashing function supported by password_verify() (among others, this includes the state of the art hashes bcrypt and Argon).

Furthermore this PR is about generalized approach on using the PDO Backend with Basic Authentication. The supplied Backend allows the customization of:

• tableName : table in which the user information is stored,
• digestColumn : table column in which the digest/passoword hash is stored,
• digestPrefix : if your user management Backend prefixes your digests, you can specify it so it will be removed before verfiying it.

I think this covers a large part of use cases and would benefit a lot of sabre/dav users.

Best Regards.

[lightbluetom]

Closing this Request, because i have some problems pushing to this PR.

Sorry for the inconvinience.