fix: escape quoted-string metacharacters in digest-md5 response

[netliomax25-code]

1. computeResponse writes username, realm, nonce and digest-uri straight into the quoted-string directives of the digest-response without escaping \ and ", which RFC 2831 section 7.1 requires for a quoted string.
2. A " in the username (application supplied) or in the server-supplied realm/nonce that gets echoed back closes the directive early and injects extra directives into the SASL credential, e.g. username a",realm="x yields a second realm= in the response.
3. Added quotedStringEscape and applied it to each quoted value. The digest is still computed over the raw values, so only the wire encoding changes and the server recovers the same values after unescaping.

Validation: added two unit tests in bind_test.go that fail on the current code (the raw quote breaks out) and pass with the escape applied.